xref: /freebsd/contrib/llvm-project/llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp (revision e32fecd0c2c3ee37c47ee100f169e7eb0282a873)
1 //===- SeparateConstOffsetFromGEP.cpp -------------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // Loop unrolling may create many similar GEPs for array accesses.
10 // e.g., a 2-level loop
11 //
12 // float a[32][32]; // global variable
13 //
14 // for (int i = 0; i < 2; ++i) {
15 //   for (int j = 0; j < 2; ++j) {
16 //     ...
17 //     ... = a[x + i][y + j];
18 //     ...
19 //   }
20 // }
21 //
22 // will probably be unrolled to:
23 //
24 // gep %a, 0, %x, %y; load
25 // gep %a, 0, %x, %y + 1; load
26 // gep %a, 0, %x + 1, %y; load
27 // gep %a, 0, %x + 1, %y + 1; load
28 //
29 // LLVM's GVN does not use partial redundancy elimination yet, and is thus
30 // unable to reuse (gep %a, 0, %x, %y). As a result, this misoptimization incurs
31 // significant slowdown in targets with limited addressing modes. For instance,
32 // because the PTX target does not support the reg+reg addressing mode, the
33 // NVPTX backend emits PTX code that literally computes the pointer address of
34 // each GEP, wasting tons of registers. It emits the following PTX for the
35 // first load and similar PTX for other loads.
36 //
37 // mov.u32         %r1, %x;
38 // mov.u32         %r2, %y;
39 // mul.wide.u32    %rl2, %r1, 128;
40 // mov.u64         %rl3, a;
41 // add.s64         %rl4, %rl3, %rl2;
42 // mul.wide.u32    %rl5, %r2, 4;
43 // add.s64         %rl6, %rl4, %rl5;
44 // ld.global.f32   %f1, [%rl6];
45 //
46 // To reduce the register pressure, the optimization implemented in this file
47 // merges the common part of a group of GEPs, so we can compute each pointer
48 // address by adding a simple offset to the common part, saving many registers.
49 //
50 // It works by splitting each GEP into a variadic base and a constant offset.
51 // The variadic base can be computed once and reused by multiple GEPs, and the
52 // constant offsets can be nicely folded into the reg+immediate addressing mode
53 // (supported by most targets) without using any extra register.
54 //
55 // For instance, we transform the four GEPs and four loads in the above example
56 // into:
57 //
58 // base = gep a, 0, x, y
59 // load base
60 // laod base + 1  * sizeof(float)
61 // load base + 32 * sizeof(float)
62 // load base + 33 * sizeof(float)
63 //
64 // Given the transformed IR, a backend that supports the reg+immediate
65 // addressing mode can easily fold the pointer arithmetics into the loads. For
66 // example, the NVPTX backend can easily fold the pointer arithmetics into the
67 // ld.global.f32 instructions, and the resultant PTX uses much fewer registers.
68 //
69 // mov.u32         %r1, %tid.x;
70 // mov.u32         %r2, %tid.y;
71 // mul.wide.u32    %rl2, %r1, 128;
72 // mov.u64         %rl3, a;
73 // add.s64         %rl4, %rl3, %rl2;
74 // mul.wide.u32    %rl5, %r2, 4;
75 // add.s64         %rl6, %rl4, %rl5;
76 // ld.global.f32   %f1, [%rl6]; // so far the same as unoptimized PTX
77 // ld.global.f32   %f2, [%rl6+4]; // much better
78 // ld.global.f32   %f3, [%rl6+128]; // much better
79 // ld.global.f32   %f4, [%rl6+132]; // much better
80 //
81 // Another improvement enabled by the LowerGEP flag is to lower a GEP with
82 // multiple indices to either multiple GEPs with a single index or arithmetic
83 // operations (depending on whether the target uses alias analysis in codegen).
84 // Such transformation can have following benefits:
85 // (1) It can always extract constants in the indices of structure type.
86 // (2) After such Lowering, there are more optimization opportunities such as
87 //     CSE, LICM and CGP.
88 //
89 // E.g. The following GEPs have multiple indices:
90 //  BB1:
91 //    %p = getelementptr [10 x %struct]* %ptr, i64 %i, i64 %j1, i32 3
92 //    load %p
93 //    ...
94 //  BB2:
95 //    %p2 = getelementptr [10 x %struct]* %ptr, i64 %i, i64 %j1, i32 2
96 //    load %p2
97 //    ...
98 //
99 // We can not do CSE to the common part related to index "i64 %i". Lowering
100 // GEPs can achieve such goals.
101 // If the target does not use alias analysis in codegen, this pass will
102 // lower a GEP with multiple indices into arithmetic operations:
103 //  BB1:
104 //    %1 = ptrtoint [10 x %struct]* %ptr to i64    ; CSE opportunity
105 //    %2 = mul i64 %i, length_of_10xstruct         ; CSE opportunity
106 //    %3 = add i64 %1, %2                          ; CSE opportunity
107 //    %4 = mul i64 %j1, length_of_struct
108 //    %5 = add i64 %3, %4
109 //    %6 = add i64 %3, struct_field_3              ; Constant offset
110 //    %p = inttoptr i64 %6 to i32*
111 //    load %p
112 //    ...
113 //  BB2:
114 //    %7 = ptrtoint [10 x %struct]* %ptr to i64    ; CSE opportunity
115 //    %8 = mul i64 %i, length_of_10xstruct         ; CSE opportunity
116 //    %9 = add i64 %7, %8                          ; CSE opportunity
117 //    %10 = mul i64 %j2, length_of_struct
118 //    %11 = add i64 %9, %10
119 //    %12 = add i64 %11, struct_field_2            ; Constant offset
120 //    %p = inttoptr i64 %12 to i32*
121 //    load %p2
122 //    ...
123 //
124 // If the target uses alias analysis in codegen, this pass will lower a GEP
125 // with multiple indices into multiple GEPs with a single index:
126 //  BB1:
127 //    %1 = bitcast [10 x %struct]* %ptr to i8*     ; CSE opportunity
128 //    %2 = mul i64 %i, length_of_10xstruct         ; CSE opportunity
129 //    %3 = getelementptr i8* %1, i64 %2            ; CSE opportunity
130 //    %4 = mul i64 %j1, length_of_struct
131 //    %5 = getelementptr i8* %3, i64 %4
132 //    %6 = getelementptr i8* %5, struct_field_3    ; Constant offset
133 //    %p = bitcast i8* %6 to i32*
134 //    load %p
135 //    ...
136 //  BB2:
137 //    %7 = bitcast [10 x %struct]* %ptr to i8*     ; CSE opportunity
138 //    %8 = mul i64 %i, length_of_10xstruct         ; CSE opportunity
139 //    %9 = getelementptr i8* %7, i64 %8            ; CSE opportunity
140 //    %10 = mul i64 %j2, length_of_struct
141 //    %11 = getelementptr i8* %9, i64 %10
142 //    %12 = getelementptr i8* %11, struct_field_2  ; Constant offset
143 //    %p2 = bitcast i8* %12 to i32*
144 //    load %p2
145 //    ...
146 //
147 // Lowering GEPs can also benefit other passes such as LICM and CGP.
148 // LICM (Loop Invariant Code Motion) can not hoist/sink a GEP of multiple
149 // indices if one of the index is variant. If we lower such GEP into invariant
150 // parts and variant parts, LICM can hoist/sink those invariant parts.
151 // CGP (CodeGen Prepare) tries to sink address calculations that match the
152 // target's addressing modes. A GEP with multiple indices may not match and will
153 // not be sunk. If we lower such GEP into smaller parts, CGP may sink some of
154 // them. So we end up with a better addressing mode.
155 //
156 //===----------------------------------------------------------------------===//
157 
158 #include "llvm/Transforms/Scalar/SeparateConstOffsetFromGEP.h"
159 #include "llvm/ADT/APInt.h"
160 #include "llvm/ADT/DenseMap.h"
161 #include "llvm/ADT/DepthFirstIterator.h"
162 #include "llvm/ADT/SmallVector.h"
163 #include "llvm/Analysis/LoopInfo.h"
164 #include "llvm/Analysis/MemoryBuiltins.h"
165 #include "llvm/Analysis/ScalarEvolution.h"
166 #include "llvm/Analysis/TargetLibraryInfo.h"
167 #include "llvm/Analysis/TargetTransformInfo.h"
168 #include "llvm/Analysis/ValueTracking.h"
169 #include "llvm/IR/BasicBlock.h"
170 #include "llvm/IR/Constant.h"
171 #include "llvm/IR/Constants.h"
172 #include "llvm/IR/DataLayout.h"
173 #include "llvm/IR/DerivedTypes.h"
174 #include "llvm/IR/Dominators.h"
175 #include "llvm/IR/Function.h"
176 #include "llvm/IR/GetElementPtrTypeIterator.h"
177 #include "llvm/IR/IRBuilder.h"
178 #include "llvm/IR/Instruction.h"
179 #include "llvm/IR/Instructions.h"
180 #include "llvm/IR/Module.h"
181 #include "llvm/IR/PassManager.h"
182 #include "llvm/IR/PatternMatch.h"
183 #include "llvm/IR/Type.h"
184 #include "llvm/IR/User.h"
185 #include "llvm/IR/Value.h"
186 #include "llvm/InitializePasses.h"
187 #include "llvm/Pass.h"
188 #include "llvm/Support/Casting.h"
189 #include "llvm/Support/CommandLine.h"
190 #include "llvm/Support/ErrorHandling.h"
191 #include "llvm/Support/raw_ostream.h"
192 #include "llvm/Transforms/Scalar.h"
193 #include "llvm/Transforms/Utils/Local.h"
194 #include <cassert>
195 #include <cstdint>
196 #include <string>
197 
198 using namespace llvm;
199 using namespace llvm::PatternMatch;
200 
201 static cl::opt<bool> DisableSeparateConstOffsetFromGEP(
202     "disable-separate-const-offset-from-gep", cl::init(false),
203     cl::desc("Do not separate the constant offset from a GEP instruction"),
204     cl::Hidden);
205 
206 // Setting this flag may emit false positives when the input module already
207 // contains dead instructions. Therefore, we set it only in unit tests that are
208 // free of dead code.
209 static cl::opt<bool>
210     VerifyNoDeadCode("reassociate-geps-verify-no-dead-code", cl::init(false),
211                      cl::desc("Verify this pass produces no dead code"),
212                      cl::Hidden);
213 
214 namespace {
215 
216 /// A helper class for separating a constant offset from a GEP index.
217 ///
218 /// In real programs, a GEP index may be more complicated than a simple addition
219 /// of something and a constant integer which can be trivially splitted. For
220 /// example, to split ((a << 3) | 5) + b, we need to search deeper for the
221 /// constant offset, so that we can separate the index to (a << 3) + b and 5.
222 ///
223 /// Therefore, this class looks into the expression that computes a given GEP
224 /// index, and tries to find a constant integer that can be hoisted to the
225 /// outermost level of the expression as an addition. Not every constant in an
226 /// expression can jump out. e.g., we cannot transform (b * (a + 5)) to (b * a +
227 /// 5); nor can we transform (3 * (a + 5)) to (3 * a + 5), however in this case,
228 /// -instcombine probably already optimized (3 * (a + 5)) to (3 * a + 15).
229 class ConstantOffsetExtractor {
230 public:
231   /// Extracts a constant offset from the given GEP index. It returns the
232   /// new index representing the remainder (equal to the original index minus
233   /// the constant offset), or nullptr if we cannot extract a constant offset.
234   /// \p Idx The given GEP index
235   /// \p GEP The given GEP
236   /// \p UserChainTail Outputs the tail of UserChain so that we can
237   ///                  garbage-collect unused instructions in UserChain.
238   static Value *Extract(Value *Idx, GetElementPtrInst *GEP,
239                         User *&UserChainTail, const DominatorTree *DT);
240 
241   /// Looks for a constant offset from the given GEP index without extracting
242   /// it. It returns the numeric value of the extracted constant offset (0 if
243   /// failed). The meaning of the arguments are the same as Extract.
244   static int64_t Find(Value *Idx, GetElementPtrInst *GEP,
245                       const DominatorTree *DT);
246 
247 private:
248   ConstantOffsetExtractor(Instruction *InsertionPt, const DominatorTree *DT)
249       : IP(InsertionPt), DL(InsertionPt->getModule()->getDataLayout()), DT(DT) {
250   }
251 
252   /// Searches the expression that computes V for a non-zero constant C s.t.
253   /// V can be reassociated into the form V' + C. If the searching is
254   /// successful, returns C and update UserChain as a def-use chain from C to V;
255   /// otherwise, UserChain is empty.
256   ///
257   /// \p V            The given expression
258   /// \p SignExtended Whether V will be sign-extended in the computation of the
259   ///                 GEP index
260   /// \p ZeroExtended Whether V will be zero-extended in the computation of the
261   ///                 GEP index
262   /// \p NonNegative  Whether V is guaranteed to be non-negative. For example,
263   ///                 an index of an inbounds GEP is guaranteed to be
264   ///                 non-negative. Levaraging this, we can better split
265   ///                 inbounds GEPs.
266   APInt find(Value *V, bool SignExtended, bool ZeroExtended, bool NonNegative);
267 
268   /// A helper function to look into both operands of a binary operator.
269   APInt findInEitherOperand(BinaryOperator *BO, bool SignExtended,
270                             bool ZeroExtended);
271 
272   /// After finding the constant offset C from the GEP index I, we build a new
273   /// index I' s.t. I' + C = I. This function builds and returns the new
274   /// index I' according to UserChain produced by function "find".
275   ///
276   /// The building conceptually takes two steps:
277   /// 1) iteratively distribute s/zext towards the leaves of the expression tree
278   /// that computes I
279   /// 2) reassociate the expression tree to the form I' + C.
280   ///
281   /// For example, to extract the 5 from sext(a + (b + 5)), we first distribute
282   /// sext to a, b and 5 so that we have
283   ///   sext(a) + (sext(b) + 5).
284   /// Then, we reassociate it to
285   ///   (sext(a) + sext(b)) + 5.
286   /// Given this form, we know I' is sext(a) + sext(b).
287   Value *rebuildWithoutConstOffset();
288 
289   /// After the first step of rebuilding the GEP index without the constant
290   /// offset, distribute s/zext to the operands of all operators in UserChain.
291   /// e.g., zext(sext(a + (b + 5)) (assuming no overflow) =>
292   /// zext(sext(a)) + (zext(sext(b)) + zext(sext(5))).
293   ///
294   /// The function also updates UserChain to point to new subexpressions after
295   /// distributing s/zext. e.g., the old UserChain of the above example is
296   /// 5 -> b + 5 -> a + (b + 5) -> sext(...) -> zext(sext(...)),
297   /// and the new UserChain is
298   /// zext(sext(5)) -> zext(sext(b)) + zext(sext(5)) ->
299   ///   zext(sext(a)) + (zext(sext(b)) + zext(sext(5))
300   ///
301   /// \p ChainIndex The index to UserChain. ChainIndex is initially
302   ///               UserChain.size() - 1, and is decremented during
303   ///               the recursion.
304   Value *distributeExtsAndCloneChain(unsigned ChainIndex);
305 
306   /// Reassociates the GEP index to the form I' + C and returns I'.
307   Value *removeConstOffset(unsigned ChainIndex);
308 
309   /// A helper function to apply ExtInsts, a list of s/zext, to value V.
310   /// e.g., if ExtInsts = [sext i32 to i64, zext i16 to i32], this function
311   /// returns "sext i32 (zext i16 V to i32) to i64".
312   Value *applyExts(Value *V);
313 
314   /// A helper function that returns whether we can trace into the operands
315   /// of binary operator BO for a constant offset.
316   ///
317   /// \p SignExtended Whether BO is surrounded by sext
318   /// \p ZeroExtended Whether BO is surrounded by zext
319   /// \p NonNegative Whether BO is known to be non-negative, e.g., an in-bound
320   ///                array index.
321   bool CanTraceInto(bool SignExtended, bool ZeroExtended, BinaryOperator *BO,
322                     bool NonNegative);
323 
324   /// The path from the constant offset to the old GEP index. e.g., if the GEP
325   /// index is "a * b + (c + 5)". After running function find, UserChain[0] will
326   /// be the constant 5, UserChain[1] will be the subexpression "c + 5", and
327   /// UserChain[2] will be the entire expression "a * b + (c + 5)".
328   ///
329   /// This path helps to rebuild the new GEP index.
330   SmallVector<User *, 8> UserChain;
331 
332   /// A data structure used in rebuildWithoutConstOffset. Contains all
333   /// sext/zext instructions along UserChain.
334   SmallVector<CastInst *, 16> ExtInsts;
335 
336   /// Insertion position of cloned instructions.
337   Instruction *IP;
338 
339   const DataLayout &DL;
340   const DominatorTree *DT;
341 };
342 
343 /// A pass that tries to split every GEP in the function into a variadic
344 /// base and a constant offset. It is a FunctionPass because searching for the
345 /// constant offset may inspect other basic blocks.
346 class SeparateConstOffsetFromGEPLegacyPass : public FunctionPass {
347 public:
348   static char ID;
349 
350   SeparateConstOffsetFromGEPLegacyPass(bool LowerGEP = false)
351       : FunctionPass(ID), LowerGEP(LowerGEP) {
352     initializeSeparateConstOffsetFromGEPLegacyPassPass(
353         *PassRegistry::getPassRegistry());
354   }
355 
356   void getAnalysisUsage(AnalysisUsage &AU) const override {
357     AU.addRequired<DominatorTreeWrapperPass>();
358     AU.addRequired<ScalarEvolutionWrapperPass>();
359     AU.addRequired<TargetTransformInfoWrapperPass>();
360     AU.addRequired<LoopInfoWrapperPass>();
361     AU.setPreservesCFG();
362     AU.addRequired<TargetLibraryInfoWrapperPass>();
363   }
364 
365   bool runOnFunction(Function &F) override;
366 
367 private:
368   bool LowerGEP;
369 };
370 
371 /// A pass that tries to split every GEP in the function into a variadic
372 /// base and a constant offset. It is a FunctionPass because searching for the
373 /// constant offset may inspect other basic blocks.
374 class SeparateConstOffsetFromGEP {
375 public:
376   SeparateConstOffsetFromGEP(
377       DominatorTree *DT, ScalarEvolution *SE, LoopInfo *LI,
378       TargetLibraryInfo *TLI,
379       function_ref<TargetTransformInfo &(Function &)> GetTTI, bool LowerGEP)
380       : DT(DT), SE(SE), LI(LI), TLI(TLI), GetTTI(GetTTI), LowerGEP(LowerGEP) {}
381 
382   bool run(Function &F);
383 
384 private:
385   /// Tries to split the given GEP into a variadic base and a constant offset,
386   /// and returns true if the splitting succeeds.
387   bool splitGEP(GetElementPtrInst *GEP);
388 
389   /// Lower a GEP with multiple indices into multiple GEPs with a single index.
390   /// Function splitGEP already split the original GEP into a variadic part and
391   /// a constant offset (i.e., AccumulativeByteOffset). This function lowers the
392   /// variadic part into a set of GEPs with a single index and applies
393   /// AccumulativeByteOffset to it.
394   /// \p Variadic                  The variadic part of the original GEP.
395   /// \p AccumulativeByteOffset    The constant offset.
396   void lowerToSingleIndexGEPs(GetElementPtrInst *Variadic,
397                               int64_t AccumulativeByteOffset);
398 
399   /// Lower a GEP with multiple indices into ptrtoint+arithmetics+inttoptr form.
400   /// Function splitGEP already split the original GEP into a variadic part and
401   /// a constant offset (i.e., AccumulativeByteOffset). This function lowers the
402   /// variadic part into a set of arithmetic operations and applies
403   /// AccumulativeByteOffset to it.
404   /// \p Variadic                  The variadic part of the original GEP.
405   /// \p AccumulativeByteOffset    The constant offset.
406   void lowerToArithmetics(GetElementPtrInst *Variadic,
407                           int64_t AccumulativeByteOffset);
408 
409   /// Finds the constant offset within each index and accumulates them. If
410   /// LowerGEP is true, it finds in indices of both sequential and structure
411   /// types, otherwise it only finds in sequential indices. The output
412   /// NeedsExtraction indicates whether we successfully find a non-zero constant
413   /// offset.
414   int64_t accumulateByteOffset(GetElementPtrInst *GEP, bool &NeedsExtraction);
415 
416   /// Canonicalize array indices to pointer-size integers. This helps to
417   /// simplify the logic of splitting a GEP. For example, if a + b is a
418   /// pointer-size integer, we have
419   ///   gep base, a + b = gep (gep base, a), b
420   /// However, this equality may not hold if the size of a + b is smaller than
421   /// the pointer size, because LLVM conceptually sign-extends GEP indices to
422   /// pointer size before computing the address
423   /// (http://llvm.org/docs/LangRef.html#id181).
424   ///
425   /// This canonicalization is very likely already done in clang and
426   /// instcombine. Therefore, the program will probably remain the same.
427   ///
428   /// Returns true if the module changes.
429   ///
430   /// Verified in @i32_add in split-gep.ll
431   bool canonicalizeArrayIndicesToPointerSize(GetElementPtrInst *GEP);
432 
433   /// Optimize sext(a)+sext(b) to sext(a+b) when a+b can't sign overflow.
434   /// SeparateConstOffsetFromGEP distributes a sext to leaves before extracting
435   /// the constant offset. After extraction, it becomes desirable to reunion the
436   /// distributed sexts. For example,
437   ///
438   ///                              &a[sext(i +nsw (j +nsw 5)]
439   ///   => distribute              &a[sext(i) +nsw (sext(j) +nsw 5)]
440   ///   => constant extraction     &a[sext(i) + sext(j)] + 5
441   ///   => reunion                 &a[sext(i +nsw j)] + 5
442   bool reuniteExts(Function &F);
443 
444   /// A helper that reunites sexts in an instruction.
445   bool reuniteExts(Instruction *I);
446 
447   /// Find the closest dominator of <Dominatee> that is equivalent to <Key>.
448   Instruction *findClosestMatchingDominator(
449       const SCEV *Key, Instruction *Dominatee,
450       DenseMap<const SCEV *, SmallVector<Instruction *, 2>> &DominatingExprs);
451 
452   /// Verify F is free of dead code.
453   void verifyNoDeadCode(Function &F);
454 
455   bool hasMoreThanOneUseInLoop(Value *v, Loop *L);
456 
457   // Swap the index operand of two GEP.
458   void swapGEPOperand(GetElementPtrInst *First, GetElementPtrInst *Second);
459 
460   // Check if it is safe to swap operand of two GEP.
461   bool isLegalToSwapOperand(GetElementPtrInst *First, GetElementPtrInst *Second,
462                             Loop *CurLoop);
463 
464   const DataLayout *DL = nullptr;
465   DominatorTree *DT = nullptr;
466   ScalarEvolution *SE;
467   LoopInfo *LI;
468   TargetLibraryInfo *TLI;
469   // Retrieved lazily since not always used.
470   function_ref<TargetTransformInfo &(Function &)> GetTTI;
471 
472   /// Whether to lower a GEP with multiple indices into arithmetic operations or
473   /// multiple GEPs with a single index.
474   bool LowerGEP;
475 
476   DenseMap<const SCEV *, SmallVector<Instruction *, 2>> DominatingAdds;
477   DenseMap<const SCEV *, SmallVector<Instruction *, 2>> DominatingSubs;
478 };
479 
480 } // end anonymous namespace
481 
482 char SeparateConstOffsetFromGEPLegacyPass::ID = 0;
483 
484 INITIALIZE_PASS_BEGIN(
485     SeparateConstOffsetFromGEPLegacyPass, "separate-const-offset-from-gep",
486     "Split GEPs to a variadic base and a constant offset for better CSE", false,
487     false)
488 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
489 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
490 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
491 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
492 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
493 INITIALIZE_PASS_END(
494     SeparateConstOffsetFromGEPLegacyPass, "separate-const-offset-from-gep",
495     "Split GEPs to a variadic base and a constant offset for better CSE", false,
496     false)
497 
498 FunctionPass *llvm::createSeparateConstOffsetFromGEPPass(bool LowerGEP) {
499   return new SeparateConstOffsetFromGEPLegacyPass(LowerGEP);
500 }
501 
502 bool ConstantOffsetExtractor::CanTraceInto(bool SignExtended,
503                                             bool ZeroExtended,
504                                             BinaryOperator *BO,
505                                             bool NonNegative) {
506   // We only consider ADD, SUB and OR, because a non-zero constant found in
507   // expressions composed of these operations can be easily hoisted as a
508   // constant offset by reassociation.
509   if (BO->getOpcode() != Instruction::Add &&
510       BO->getOpcode() != Instruction::Sub &&
511       BO->getOpcode() != Instruction::Or) {
512     return false;
513   }
514 
515   Value *LHS = BO->getOperand(0), *RHS = BO->getOperand(1);
516   // Do not trace into "or" unless it is equivalent to "add". If LHS and RHS
517   // don't have common bits, (LHS | RHS) is equivalent to (LHS + RHS).
518   // FIXME: this does not appear to be covered by any tests
519   //        (with x86/aarch64 backends at least)
520   if (BO->getOpcode() == Instruction::Or &&
521       !haveNoCommonBitsSet(LHS, RHS, DL, nullptr, BO, DT))
522     return false;
523 
524   // In addition, tracing into BO requires that its surrounding s/zext (if
525   // any) is distributable to both operands.
526   //
527   // Suppose BO = A op B.
528   //  SignExtended | ZeroExtended | Distributable?
529   // --------------+--------------+----------------------------------
530   //       0       |      0       | true because no s/zext exists
531   //       0       |      1       | zext(BO) == zext(A) op zext(B)
532   //       1       |      0       | sext(BO) == sext(A) op sext(B)
533   //       1       |      1       | zext(sext(BO)) ==
534   //               |              |     zext(sext(A)) op zext(sext(B))
535   if (BO->getOpcode() == Instruction::Add && !ZeroExtended && NonNegative) {
536     // If a + b >= 0 and (a >= 0 or b >= 0), then
537     //   sext(a + b) = sext(a) + sext(b)
538     // even if the addition is not marked nsw.
539     //
540     // Leveraging this invariant, we can trace into an sext'ed inbound GEP
541     // index if the constant offset is non-negative.
542     //
543     // Verified in @sext_add in split-gep.ll.
544     if (ConstantInt *ConstLHS = dyn_cast<ConstantInt>(LHS)) {
545       if (!ConstLHS->isNegative())
546         return true;
547     }
548     if (ConstantInt *ConstRHS = dyn_cast<ConstantInt>(RHS)) {
549       if (!ConstRHS->isNegative())
550         return true;
551     }
552   }
553 
554   // sext (add/sub nsw A, B) == add/sub nsw (sext A), (sext B)
555   // zext (add/sub nuw A, B) == add/sub nuw (zext A), (zext B)
556   if (BO->getOpcode() == Instruction::Add ||
557       BO->getOpcode() == Instruction::Sub) {
558     if (SignExtended && !BO->hasNoSignedWrap())
559       return false;
560     if (ZeroExtended && !BO->hasNoUnsignedWrap())
561       return false;
562   }
563 
564   return true;
565 }
566 
567 APInt ConstantOffsetExtractor::findInEitherOperand(BinaryOperator *BO,
568                                                    bool SignExtended,
569                                                    bool ZeroExtended) {
570   // Save off the current height of the chain, in case we need to restore it.
571   size_t ChainLength = UserChain.size();
572 
573   // BO being non-negative does not shed light on whether its operands are
574   // non-negative. Clear the NonNegative flag here.
575   APInt ConstantOffset = find(BO->getOperand(0), SignExtended, ZeroExtended,
576                               /* NonNegative */ false);
577   // If we found a constant offset in the left operand, stop and return that.
578   // This shortcut might cause us to miss opportunities of combining the
579   // constant offsets in both operands, e.g., (a + 4) + (b + 5) => (a + b) + 9.
580   // However, such cases are probably already handled by -instcombine,
581   // given this pass runs after the standard optimizations.
582   if (ConstantOffset != 0) return ConstantOffset;
583 
584   // Reset the chain back to where it was when we started exploring this node,
585   // since visiting the LHS didn't pan out.
586   UserChain.resize(ChainLength);
587 
588   ConstantOffset = find(BO->getOperand(1), SignExtended, ZeroExtended,
589                         /* NonNegative */ false);
590   // If U is a sub operator, negate the constant offset found in the right
591   // operand.
592   if (BO->getOpcode() == Instruction::Sub)
593     ConstantOffset = -ConstantOffset;
594 
595   // If RHS wasn't a suitable candidate either, reset the chain again.
596   if (ConstantOffset == 0)
597     UserChain.resize(ChainLength);
598 
599   return ConstantOffset;
600 }
601 
602 APInt ConstantOffsetExtractor::find(Value *V, bool SignExtended,
603                                     bool ZeroExtended, bool NonNegative) {
604   // TODO(jingyue): We could trace into integer/pointer casts, such as
605   // inttoptr, ptrtoint, bitcast, and addrspacecast. We choose to handle only
606   // integers because it gives good enough results for our benchmarks.
607   unsigned BitWidth = cast<IntegerType>(V->getType())->getBitWidth();
608 
609   // We cannot do much with Values that are not a User, such as an Argument.
610   User *U = dyn_cast<User>(V);
611   if (U == nullptr) return APInt(BitWidth, 0);
612 
613   APInt ConstantOffset(BitWidth, 0);
614   if (ConstantInt *CI = dyn_cast<ConstantInt>(V)) {
615     // Hooray, we found it!
616     ConstantOffset = CI->getValue();
617   } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(V)) {
618     // Trace into subexpressions for more hoisting opportunities.
619     if (CanTraceInto(SignExtended, ZeroExtended, BO, NonNegative))
620       ConstantOffset = findInEitherOperand(BO, SignExtended, ZeroExtended);
621   } else if (isa<TruncInst>(V)) {
622     ConstantOffset =
623         find(U->getOperand(0), SignExtended, ZeroExtended, NonNegative)
624             .trunc(BitWidth);
625   } else if (isa<SExtInst>(V)) {
626     ConstantOffset = find(U->getOperand(0), /* SignExtended */ true,
627                           ZeroExtended, NonNegative).sext(BitWidth);
628   } else if (isa<ZExtInst>(V)) {
629     // As an optimization, we can clear the SignExtended flag because
630     // sext(zext(a)) = zext(a). Verified in @sext_zext in split-gep.ll.
631     //
632     // Clear the NonNegative flag, because zext(a) >= 0 does not imply a >= 0.
633     ConstantOffset =
634         find(U->getOperand(0), /* SignExtended */ false,
635              /* ZeroExtended */ true, /* NonNegative */ false).zext(BitWidth);
636   }
637 
638   // If we found a non-zero constant offset, add it to the path for
639   // rebuildWithoutConstOffset. Zero is a valid constant offset, but doesn't
640   // help this optimization.
641   if (ConstantOffset != 0)
642     UserChain.push_back(U);
643   return ConstantOffset;
644 }
645 
646 Value *ConstantOffsetExtractor::applyExts(Value *V) {
647   Value *Current = V;
648   // ExtInsts is built in the use-def order. Therefore, we apply them to V
649   // in the reversed order.
650   for (CastInst *I : llvm::reverse(ExtInsts)) {
651     if (Constant *C = dyn_cast<Constant>(Current)) {
652       // If Current is a constant, apply s/zext using ConstantExpr::getCast.
653       // ConstantExpr::getCast emits a ConstantInt if C is a ConstantInt.
654       Current = ConstantExpr::getCast(I->getOpcode(), C, I->getType());
655     } else {
656       Instruction *Ext = I->clone();
657       Ext->setOperand(0, Current);
658       Ext->insertBefore(IP);
659       Current = Ext;
660     }
661   }
662   return Current;
663 }
664 
665 Value *ConstantOffsetExtractor::rebuildWithoutConstOffset() {
666   distributeExtsAndCloneChain(UserChain.size() - 1);
667   // Remove all nullptrs (used to be s/zext) from UserChain.
668   unsigned NewSize = 0;
669   for (User *I : UserChain) {
670     if (I != nullptr) {
671       UserChain[NewSize] = I;
672       NewSize++;
673     }
674   }
675   UserChain.resize(NewSize);
676   return removeConstOffset(UserChain.size() - 1);
677 }
678 
679 Value *
680 ConstantOffsetExtractor::distributeExtsAndCloneChain(unsigned ChainIndex) {
681   User *U = UserChain[ChainIndex];
682   if (ChainIndex == 0) {
683     assert(isa<ConstantInt>(U));
684     // If U is a ConstantInt, applyExts will return a ConstantInt as well.
685     return UserChain[ChainIndex] = cast<ConstantInt>(applyExts(U));
686   }
687 
688   if (CastInst *Cast = dyn_cast<CastInst>(U)) {
689     assert(
690         (isa<SExtInst>(Cast) || isa<ZExtInst>(Cast) || isa<TruncInst>(Cast)) &&
691         "Only following instructions can be traced: sext, zext & trunc");
692     ExtInsts.push_back(Cast);
693     UserChain[ChainIndex] = nullptr;
694     return distributeExtsAndCloneChain(ChainIndex - 1);
695   }
696 
697   // Function find only trace into BinaryOperator and CastInst.
698   BinaryOperator *BO = cast<BinaryOperator>(U);
699   // OpNo = which operand of BO is UserChain[ChainIndex - 1]
700   unsigned OpNo = (BO->getOperand(0) == UserChain[ChainIndex - 1] ? 0 : 1);
701   Value *TheOther = applyExts(BO->getOperand(1 - OpNo));
702   Value *NextInChain = distributeExtsAndCloneChain(ChainIndex - 1);
703 
704   BinaryOperator *NewBO = nullptr;
705   if (OpNo == 0) {
706     NewBO = BinaryOperator::Create(BO->getOpcode(), NextInChain, TheOther,
707                                    BO->getName(), IP);
708   } else {
709     NewBO = BinaryOperator::Create(BO->getOpcode(), TheOther, NextInChain,
710                                    BO->getName(), IP);
711   }
712   return UserChain[ChainIndex] = NewBO;
713 }
714 
715 Value *ConstantOffsetExtractor::removeConstOffset(unsigned ChainIndex) {
716   if (ChainIndex == 0) {
717     assert(isa<ConstantInt>(UserChain[ChainIndex]));
718     return ConstantInt::getNullValue(UserChain[ChainIndex]->getType());
719   }
720 
721   BinaryOperator *BO = cast<BinaryOperator>(UserChain[ChainIndex]);
722   assert((BO->use_empty() || BO->hasOneUse()) &&
723          "distributeExtsAndCloneChain clones each BinaryOperator in "
724          "UserChain, so no one should be used more than "
725          "once");
726 
727   unsigned OpNo = (BO->getOperand(0) == UserChain[ChainIndex - 1] ? 0 : 1);
728   assert(BO->getOperand(OpNo) == UserChain[ChainIndex - 1]);
729   Value *NextInChain = removeConstOffset(ChainIndex - 1);
730   Value *TheOther = BO->getOperand(1 - OpNo);
731 
732   // If NextInChain is 0 and not the LHS of a sub, we can simplify the
733   // sub-expression to be just TheOther.
734   if (ConstantInt *CI = dyn_cast<ConstantInt>(NextInChain)) {
735     if (CI->isZero() && !(BO->getOpcode() == Instruction::Sub && OpNo == 0))
736       return TheOther;
737   }
738 
739   BinaryOperator::BinaryOps NewOp = BO->getOpcode();
740   if (BO->getOpcode() == Instruction::Or) {
741     // Rebuild "or" as "add", because "or" may be invalid for the new
742     // expression.
743     //
744     // For instance, given
745     //   a | (b + 5) where a and b + 5 have no common bits,
746     // we can extract 5 as the constant offset.
747     //
748     // However, reusing the "or" in the new index would give us
749     //   (a | b) + 5
750     // which does not equal a | (b + 5).
751     //
752     // Replacing the "or" with "add" is fine, because
753     //   a | (b + 5) = a + (b + 5) = (a + b) + 5
754     NewOp = Instruction::Add;
755   }
756 
757   BinaryOperator *NewBO;
758   if (OpNo == 0) {
759     NewBO = BinaryOperator::Create(NewOp, NextInChain, TheOther, "", IP);
760   } else {
761     NewBO = BinaryOperator::Create(NewOp, TheOther, NextInChain, "", IP);
762   }
763   NewBO->takeName(BO);
764   return NewBO;
765 }
766 
767 Value *ConstantOffsetExtractor::Extract(Value *Idx, GetElementPtrInst *GEP,
768                                         User *&UserChainTail,
769                                         const DominatorTree *DT) {
770   ConstantOffsetExtractor Extractor(GEP, DT);
771   // Find a non-zero constant offset first.
772   APInt ConstantOffset =
773       Extractor.find(Idx, /* SignExtended */ false, /* ZeroExtended */ false,
774                      GEP->isInBounds());
775   if (ConstantOffset == 0) {
776     UserChainTail = nullptr;
777     return nullptr;
778   }
779   // Separates the constant offset from the GEP index.
780   Value *IdxWithoutConstOffset = Extractor.rebuildWithoutConstOffset();
781   UserChainTail = Extractor.UserChain.back();
782   return IdxWithoutConstOffset;
783 }
784 
785 int64_t ConstantOffsetExtractor::Find(Value *Idx, GetElementPtrInst *GEP,
786                                       const DominatorTree *DT) {
787   // If Idx is an index of an inbound GEP, Idx is guaranteed to be non-negative.
788   return ConstantOffsetExtractor(GEP, DT)
789       .find(Idx, /* SignExtended */ false, /* ZeroExtended */ false,
790             GEP->isInBounds())
791       .getSExtValue();
792 }
793 
794 bool SeparateConstOffsetFromGEP::canonicalizeArrayIndicesToPointerSize(
795     GetElementPtrInst *GEP) {
796   bool Changed = false;
797   Type *IntPtrTy = DL->getIntPtrType(GEP->getType());
798   gep_type_iterator GTI = gep_type_begin(*GEP);
799   for (User::op_iterator I = GEP->op_begin() + 1, E = GEP->op_end();
800        I != E; ++I, ++GTI) {
801     // Skip struct member indices which must be i32.
802     if (GTI.isSequential()) {
803       if ((*I)->getType() != IntPtrTy) {
804         *I = CastInst::CreateIntegerCast(*I, IntPtrTy, true, "idxprom", GEP);
805         Changed = true;
806       }
807     }
808   }
809   return Changed;
810 }
811 
812 int64_t
813 SeparateConstOffsetFromGEP::accumulateByteOffset(GetElementPtrInst *GEP,
814                                                  bool &NeedsExtraction) {
815   NeedsExtraction = false;
816   int64_t AccumulativeByteOffset = 0;
817   gep_type_iterator GTI = gep_type_begin(*GEP);
818   for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I, ++GTI) {
819     if (GTI.isSequential()) {
820       // Tries to extract a constant offset from this GEP index.
821       int64_t ConstantOffset =
822           ConstantOffsetExtractor::Find(GEP->getOperand(I), GEP, DT);
823       if (ConstantOffset != 0) {
824         NeedsExtraction = true;
825         // A GEP may have multiple indices.  We accumulate the extracted
826         // constant offset to a byte offset, and later offset the remainder of
827         // the original GEP with this byte offset.
828         AccumulativeByteOffset +=
829             ConstantOffset * DL->getTypeAllocSize(GTI.getIndexedType());
830       }
831     } else if (LowerGEP) {
832       StructType *StTy = GTI.getStructType();
833       uint64_t Field = cast<ConstantInt>(GEP->getOperand(I))->getZExtValue();
834       // Skip field 0 as the offset is always 0.
835       if (Field != 0) {
836         NeedsExtraction = true;
837         AccumulativeByteOffset +=
838             DL->getStructLayout(StTy)->getElementOffset(Field);
839       }
840     }
841   }
842   return AccumulativeByteOffset;
843 }
844 
845 void SeparateConstOffsetFromGEP::lowerToSingleIndexGEPs(
846     GetElementPtrInst *Variadic, int64_t AccumulativeByteOffset) {
847   IRBuilder<> Builder(Variadic);
848   Type *IntPtrTy = DL->getIntPtrType(Variadic->getType());
849 
850   Type *I8PtrTy =
851       Builder.getInt8PtrTy(Variadic->getType()->getPointerAddressSpace());
852   Value *ResultPtr = Variadic->getOperand(0);
853   Loop *L = LI->getLoopFor(Variadic->getParent());
854   // Check if the base is not loop invariant or used more than once.
855   bool isSwapCandidate =
856       L && L->isLoopInvariant(ResultPtr) &&
857       !hasMoreThanOneUseInLoop(ResultPtr, L);
858   Value *FirstResult = nullptr;
859 
860   if (ResultPtr->getType() != I8PtrTy)
861     ResultPtr = Builder.CreateBitCast(ResultPtr, I8PtrTy);
862 
863   gep_type_iterator GTI = gep_type_begin(*Variadic);
864   // Create an ugly GEP for each sequential index. We don't create GEPs for
865   // structure indices, as they are accumulated in the constant offset index.
866   for (unsigned I = 1, E = Variadic->getNumOperands(); I != E; ++I, ++GTI) {
867     if (GTI.isSequential()) {
868       Value *Idx = Variadic->getOperand(I);
869       // Skip zero indices.
870       if (ConstantInt *CI = dyn_cast<ConstantInt>(Idx))
871         if (CI->isZero())
872           continue;
873 
874       APInt ElementSize = APInt(IntPtrTy->getIntegerBitWidth(),
875                                 DL->getTypeAllocSize(GTI.getIndexedType()));
876       // Scale the index by element size.
877       if (ElementSize != 1) {
878         if (ElementSize.isPowerOf2()) {
879           Idx = Builder.CreateShl(
880               Idx, ConstantInt::get(IntPtrTy, ElementSize.logBase2()));
881         } else {
882           Idx = Builder.CreateMul(Idx, ConstantInt::get(IntPtrTy, ElementSize));
883         }
884       }
885       // Create an ugly GEP with a single index for each index.
886       ResultPtr =
887           Builder.CreateGEP(Builder.getInt8Ty(), ResultPtr, Idx, "uglygep");
888       if (FirstResult == nullptr)
889         FirstResult = ResultPtr;
890     }
891   }
892 
893   // Create a GEP with the constant offset index.
894   if (AccumulativeByteOffset != 0) {
895     Value *Offset = ConstantInt::get(IntPtrTy, AccumulativeByteOffset);
896     ResultPtr =
897         Builder.CreateGEP(Builder.getInt8Ty(), ResultPtr, Offset, "uglygep");
898   } else
899     isSwapCandidate = false;
900 
901   // If we created a GEP with constant index, and the base is loop invariant,
902   // then we swap the first one with it, so LICM can move constant GEP out
903   // later.
904   auto *FirstGEP = dyn_cast_or_null<GetElementPtrInst>(FirstResult);
905   auto *SecondGEP = dyn_cast<GetElementPtrInst>(ResultPtr);
906   if (isSwapCandidate && isLegalToSwapOperand(FirstGEP, SecondGEP, L))
907     swapGEPOperand(FirstGEP, SecondGEP);
908 
909   if (ResultPtr->getType() != Variadic->getType())
910     ResultPtr = Builder.CreateBitCast(ResultPtr, Variadic->getType());
911 
912   Variadic->replaceAllUsesWith(ResultPtr);
913   Variadic->eraseFromParent();
914 }
915 
916 void
917 SeparateConstOffsetFromGEP::lowerToArithmetics(GetElementPtrInst *Variadic,
918                                                int64_t AccumulativeByteOffset) {
919   IRBuilder<> Builder(Variadic);
920   Type *IntPtrTy = DL->getIntPtrType(Variadic->getType());
921 
922   Value *ResultPtr = Builder.CreatePtrToInt(Variadic->getOperand(0), IntPtrTy);
923   gep_type_iterator GTI = gep_type_begin(*Variadic);
924   // Create ADD/SHL/MUL arithmetic operations for each sequential indices. We
925   // don't create arithmetics for structure indices, as they are accumulated
926   // in the constant offset index.
927   for (unsigned I = 1, E = Variadic->getNumOperands(); I != E; ++I, ++GTI) {
928     if (GTI.isSequential()) {
929       Value *Idx = Variadic->getOperand(I);
930       // Skip zero indices.
931       if (ConstantInt *CI = dyn_cast<ConstantInt>(Idx))
932         if (CI->isZero())
933           continue;
934 
935       APInt ElementSize = APInt(IntPtrTy->getIntegerBitWidth(),
936                                 DL->getTypeAllocSize(GTI.getIndexedType()));
937       // Scale the index by element size.
938       if (ElementSize != 1) {
939         if (ElementSize.isPowerOf2()) {
940           Idx = Builder.CreateShl(
941               Idx, ConstantInt::get(IntPtrTy, ElementSize.logBase2()));
942         } else {
943           Idx = Builder.CreateMul(Idx, ConstantInt::get(IntPtrTy, ElementSize));
944         }
945       }
946       // Create an ADD for each index.
947       ResultPtr = Builder.CreateAdd(ResultPtr, Idx);
948     }
949   }
950 
951   // Create an ADD for the constant offset index.
952   if (AccumulativeByteOffset != 0) {
953     ResultPtr = Builder.CreateAdd(
954         ResultPtr, ConstantInt::get(IntPtrTy, AccumulativeByteOffset));
955   }
956 
957   ResultPtr = Builder.CreateIntToPtr(ResultPtr, Variadic->getType());
958   Variadic->replaceAllUsesWith(ResultPtr);
959   Variadic->eraseFromParent();
960 }
961 
962 bool SeparateConstOffsetFromGEP::splitGEP(GetElementPtrInst *GEP) {
963   // Skip vector GEPs.
964   if (GEP->getType()->isVectorTy())
965     return false;
966 
967   // The backend can already nicely handle the case where all indices are
968   // constant.
969   if (GEP->hasAllConstantIndices())
970     return false;
971 
972   bool Changed = canonicalizeArrayIndicesToPointerSize(GEP);
973 
974   bool NeedsExtraction;
975   int64_t AccumulativeByteOffset = accumulateByteOffset(GEP, NeedsExtraction);
976 
977   if (!NeedsExtraction)
978     return Changed;
979 
980   TargetTransformInfo &TTI = GetTTI(*GEP->getFunction());
981 
982   // If LowerGEP is disabled, before really splitting the GEP, check whether the
983   // backend supports the addressing mode we are about to produce. If no, this
984   // splitting probably won't be beneficial.
985   // If LowerGEP is enabled, even the extracted constant offset can not match
986   // the addressing mode, we can still do optimizations to other lowered parts
987   // of variable indices. Therefore, we don't check for addressing modes in that
988   // case.
989   if (!LowerGEP) {
990     unsigned AddrSpace = GEP->getPointerAddressSpace();
991     if (!TTI.isLegalAddressingMode(GEP->getResultElementType(),
992                                    /*BaseGV=*/nullptr, AccumulativeByteOffset,
993                                    /*HasBaseReg=*/true, /*Scale=*/0,
994                                    AddrSpace)) {
995       return Changed;
996     }
997   }
998 
999   // Remove the constant offset in each sequential index. The resultant GEP
1000   // computes the variadic base.
1001   // Notice that we don't remove struct field indices here. If LowerGEP is
1002   // disabled, a structure index is not accumulated and we still use the old
1003   // one. If LowerGEP is enabled, a structure index is accumulated in the
1004   // constant offset. LowerToSingleIndexGEPs or lowerToArithmetics will later
1005   // handle the constant offset and won't need a new structure index.
1006   gep_type_iterator GTI = gep_type_begin(*GEP);
1007   for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I, ++GTI) {
1008     if (GTI.isSequential()) {
1009       // Splits this GEP index into a variadic part and a constant offset, and
1010       // uses the variadic part as the new index.
1011       Value *OldIdx = GEP->getOperand(I);
1012       User *UserChainTail;
1013       Value *NewIdx =
1014           ConstantOffsetExtractor::Extract(OldIdx, GEP, UserChainTail, DT);
1015       if (NewIdx != nullptr) {
1016         // Switches to the index with the constant offset removed.
1017         GEP->setOperand(I, NewIdx);
1018         // After switching to the new index, we can garbage-collect UserChain
1019         // and the old index if they are not used.
1020         RecursivelyDeleteTriviallyDeadInstructions(UserChainTail);
1021         RecursivelyDeleteTriviallyDeadInstructions(OldIdx);
1022       }
1023     }
1024   }
1025 
1026   // Clear the inbounds attribute because the new index may be off-bound.
1027   // e.g.,
1028   //
1029   //   b     = add i64 a, 5
1030   //   addr  = gep inbounds float, float* p, i64 b
1031   //
1032   // is transformed to:
1033   //
1034   //   addr2 = gep float, float* p, i64 a ; inbounds removed
1035   //   addr  = gep inbounds float, float* addr2, i64 5
1036   //
1037   // If a is -4, although the old index b is in bounds, the new index a is
1038   // off-bound. http://llvm.org/docs/LangRef.html#id181 says "if the
1039   // inbounds keyword is not present, the offsets are added to the base
1040   // address with silently-wrapping two's complement arithmetic".
1041   // Therefore, the final code will be a semantically equivalent.
1042   //
1043   // TODO(jingyue): do some range analysis to keep as many inbounds as
1044   // possible. GEPs with inbounds are more friendly to alias analysis.
1045   bool GEPWasInBounds = GEP->isInBounds();
1046   GEP->setIsInBounds(false);
1047 
1048   // Lowers a GEP to either GEPs with a single index or arithmetic operations.
1049   if (LowerGEP) {
1050     // As currently BasicAA does not analyze ptrtoint/inttoptr, do not lower to
1051     // arithmetic operations if the target uses alias analysis in codegen.
1052     if (TTI.useAA())
1053       lowerToSingleIndexGEPs(GEP, AccumulativeByteOffset);
1054     else
1055       lowerToArithmetics(GEP, AccumulativeByteOffset);
1056     return true;
1057   }
1058 
1059   // No need to create another GEP if the accumulative byte offset is 0.
1060   if (AccumulativeByteOffset == 0)
1061     return true;
1062 
1063   // Offsets the base with the accumulative byte offset.
1064   //
1065   //   %gep                        ; the base
1066   //   ... %gep ...
1067   //
1068   // => add the offset
1069   //
1070   //   %gep2                       ; clone of %gep
1071   //   %new.gep = gep %gep2, <offset / sizeof(*%gep)>
1072   //   %gep                        ; will be removed
1073   //   ... %gep ...
1074   //
1075   // => replace all uses of %gep with %new.gep and remove %gep
1076   //
1077   //   %gep2                       ; clone of %gep
1078   //   %new.gep = gep %gep2, <offset / sizeof(*%gep)>
1079   //   ... %new.gep ...
1080   //
1081   // If AccumulativeByteOffset is not a multiple of sizeof(*%gep), we emit an
1082   // uglygep (http://llvm.org/docs/GetElementPtr.html#what-s-an-uglygep):
1083   // bitcast %gep2 to i8*, add the offset, and bitcast the result back to the
1084   // type of %gep.
1085   //
1086   //   %gep2                       ; clone of %gep
1087   //   %0       = bitcast %gep2 to i8*
1088   //   %uglygep = gep %0, <offset>
1089   //   %new.gep = bitcast %uglygep to <type of %gep>
1090   //   ... %new.gep ...
1091   Instruction *NewGEP = GEP->clone();
1092   NewGEP->insertBefore(GEP);
1093 
1094   // Per ANSI C standard, signed / unsigned = unsigned and signed % unsigned =
1095   // unsigned.. Therefore, we cast ElementTypeSizeOfGEP to signed because it is
1096   // used with unsigned integers later.
1097   int64_t ElementTypeSizeOfGEP = static_cast<int64_t>(
1098       DL->getTypeAllocSize(GEP->getResultElementType()));
1099   Type *IntPtrTy = DL->getIntPtrType(GEP->getType());
1100   if (AccumulativeByteOffset % ElementTypeSizeOfGEP == 0) {
1101     // Very likely. As long as %gep is naturally aligned, the byte offset we
1102     // extracted should be a multiple of sizeof(*%gep).
1103     int64_t Index = AccumulativeByteOffset / ElementTypeSizeOfGEP;
1104     NewGEP = GetElementPtrInst::Create(GEP->getResultElementType(), NewGEP,
1105                                        ConstantInt::get(IntPtrTy, Index, true),
1106                                        GEP->getName(), GEP);
1107     NewGEP->copyMetadata(*GEP);
1108     // Inherit the inbounds attribute of the original GEP.
1109     cast<GetElementPtrInst>(NewGEP)->setIsInBounds(GEPWasInBounds);
1110   } else {
1111     // Unlikely but possible. For example,
1112     // #pragma pack(1)
1113     // struct S {
1114     //   int a[3];
1115     //   int64 b[8];
1116     // };
1117     // #pragma pack()
1118     //
1119     // Suppose the gep before extraction is &s[i + 1].b[j + 3]. After
1120     // extraction, it becomes &s[i].b[j] and AccumulativeByteOffset is
1121     // sizeof(S) + 3 * sizeof(int64) = 100, which is not a multiple of
1122     // sizeof(int64).
1123     //
1124     // Emit an uglygep in this case.
1125     Type *I8PtrTy = Type::getInt8PtrTy(GEP->getContext(),
1126                                        GEP->getPointerAddressSpace());
1127     NewGEP = new BitCastInst(NewGEP, I8PtrTy, "", GEP);
1128     NewGEP = GetElementPtrInst::Create(
1129         Type::getInt8Ty(GEP->getContext()), NewGEP,
1130         ConstantInt::get(IntPtrTy, AccumulativeByteOffset, true), "uglygep",
1131         GEP);
1132     NewGEP->copyMetadata(*GEP);
1133     // Inherit the inbounds attribute of the original GEP.
1134     cast<GetElementPtrInst>(NewGEP)->setIsInBounds(GEPWasInBounds);
1135     if (GEP->getType() != I8PtrTy)
1136       NewGEP = new BitCastInst(NewGEP, GEP->getType(), GEP->getName(), GEP);
1137   }
1138 
1139   GEP->replaceAllUsesWith(NewGEP);
1140   GEP->eraseFromParent();
1141 
1142   return true;
1143 }
1144 
1145 bool SeparateConstOffsetFromGEPLegacyPass::runOnFunction(Function &F) {
1146   if (skipFunction(F))
1147     return false;
1148   auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
1149   auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
1150   auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
1151   auto *TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
1152   auto GetTTI = [this](Function &F) -> TargetTransformInfo & {
1153     return this->getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
1154   };
1155   SeparateConstOffsetFromGEP Impl(DT, SE, LI, TLI, GetTTI, LowerGEP);
1156   return Impl.run(F);
1157 }
1158 
1159 bool SeparateConstOffsetFromGEP::run(Function &F) {
1160   if (DisableSeparateConstOffsetFromGEP)
1161     return false;
1162 
1163   DL = &F.getParent()->getDataLayout();
1164   bool Changed = false;
1165   for (BasicBlock &B : F) {
1166     if (!DT->isReachableFromEntry(&B))
1167       continue;
1168 
1169     for (Instruction &I : llvm::make_early_inc_range(B))
1170       if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(&I))
1171         Changed |= splitGEP(GEP);
1172     // No need to split GEP ConstantExprs because all its indices are constant
1173     // already.
1174   }
1175 
1176   Changed |= reuniteExts(F);
1177 
1178   if (VerifyNoDeadCode)
1179     verifyNoDeadCode(F);
1180 
1181   return Changed;
1182 }
1183 
1184 Instruction *SeparateConstOffsetFromGEP::findClosestMatchingDominator(
1185     const SCEV *Key, Instruction *Dominatee,
1186     DenseMap<const SCEV *, SmallVector<Instruction *, 2>> &DominatingExprs) {
1187   auto Pos = DominatingExprs.find(Key);
1188   if (Pos == DominatingExprs.end())
1189     return nullptr;
1190 
1191   auto &Candidates = Pos->second;
1192   // Because we process the basic blocks in pre-order of the dominator tree, a
1193   // candidate that doesn't dominate the current instruction won't dominate any
1194   // future instruction either. Therefore, we pop it out of the stack. This
1195   // optimization makes the algorithm O(n).
1196   while (!Candidates.empty()) {
1197     Instruction *Candidate = Candidates.back();
1198     if (DT->dominates(Candidate, Dominatee))
1199       return Candidate;
1200     Candidates.pop_back();
1201   }
1202   return nullptr;
1203 }
1204 
1205 bool SeparateConstOffsetFromGEP::reuniteExts(Instruction *I) {
1206   if (!SE->isSCEVable(I->getType()))
1207     return false;
1208 
1209   //   Dom: LHS+RHS
1210   //   I: sext(LHS)+sext(RHS)
1211   // If Dom can't sign overflow and Dom dominates I, optimize I to sext(Dom).
1212   // TODO: handle zext
1213   Value *LHS = nullptr, *RHS = nullptr;
1214   if (match(I, m_Add(m_SExt(m_Value(LHS)), m_SExt(m_Value(RHS))))) {
1215     if (LHS->getType() == RHS->getType()) {
1216       const SCEV *Key =
1217           SE->getAddExpr(SE->getUnknown(LHS), SE->getUnknown(RHS));
1218       if (auto *Dom = findClosestMatchingDominator(Key, I, DominatingAdds)) {
1219         Instruction *NewSExt = new SExtInst(Dom, I->getType(), "", I);
1220         NewSExt->takeName(I);
1221         I->replaceAllUsesWith(NewSExt);
1222         RecursivelyDeleteTriviallyDeadInstructions(I);
1223         return true;
1224       }
1225     }
1226   } else if (match(I, m_Sub(m_SExt(m_Value(LHS)), m_SExt(m_Value(RHS))))) {
1227     if (LHS->getType() == RHS->getType()) {
1228       const SCEV *Key =
1229           SE->getAddExpr(SE->getUnknown(LHS), SE->getUnknown(RHS));
1230       if (auto *Dom = findClosestMatchingDominator(Key, I, DominatingSubs)) {
1231         Instruction *NewSExt = new SExtInst(Dom, I->getType(), "", I);
1232         NewSExt->takeName(I);
1233         I->replaceAllUsesWith(NewSExt);
1234         RecursivelyDeleteTriviallyDeadInstructions(I);
1235         return true;
1236       }
1237     }
1238   }
1239 
1240   // Add I to DominatingExprs if it's an add/sub that can't sign overflow.
1241   if (match(I, m_NSWAdd(m_Value(LHS), m_Value(RHS)))) {
1242     if (programUndefinedIfPoison(I)) {
1243       const SCEV *Key =
1244           SE->getAddExpr(SE->getUnknown(LHS), SE->getUnknown(RHS));
1245       DominatingAdds[Key].push_back(I);
1246     }
1247   } else if (match(I, m_NSWSub(m_Value(LHS), m_Value(RHS)))) {
1248     if (programUndefinedIfPoison(I)) {
1249       const SCEV *Key =
1250           SE->getAddExpr(SE->getUnknown(LHS), SE->getUnknown(RHS));
1251       DominatingSubs[Key].push_back(I);
1252     }
1253   }
1254   return false;
1255 }
1256 
1257 bool SeparateConstOffsetFromGEP::reuniteExts(Function &F) {
1258   bool Changed = false;
1259   DominatingAdds.clear();
1260   DominatingSubs.clear();
1261   for (const auto Node : depth_first(DT)) {
1262     BasicBlock *BB = Node->getBlock();
1263     for (Instruction &I : llvm::make_early_inc_range(*BB))
1264       Changed |= reuniteExts(&I);
1265   }
1266   return Changed;
1267 }
1268 
1269 void SeparateConstOffsetFromGEP::verifyNoDeadCode(Function &F) {
1270   for (BasicBlock &B : F) {
1271     for (Instruction &I : B) {
1272       if (isInstructionTriviallyDead(&I)) {
1273         std::string ErrMessage;
1274         raw_string_ostream RSO(ErrMessage);
1275         RSO << "Dead instruction detected!\n" << I << "\n";
1276         llvm_unreachable(RSO.str().c_str());
1277       }
1278     }
1279   }
1280 }
1281 
1282 bool SeparateConstOffsetFromGEP::isLegalToSwapOperand(
1283     GetElementPtrInst *FirstGEP, GetElementPtrInst *SecondGEP, Loop *CurLoop) {
1284   if (!FirstGEP || !FirstGEP->hasOneUse())
1285     return false;
1286 
1287   if (!SecondGEP || FirstGEP->getParent() != SecondGEP->getParent())
1288     return false;
1289 
1290   if (FirstGEP == SecondGEP)
1291     return false;
1292 
1293   unsigned FirstNum = FirstGEP->getNumOperands();
1294   unsigned SecondNum = SecondGEP->getNumOperands();
1295   // Give up if the number of operands are not 2.
1296   if (FirstNum != SecondNum || FirstNum != 2)
1297     return false;
1298 
1299   Value *FirstBase = FirstGEP->getOperand(0);
1300   Value *SecondBase = SecondGEP->getOperand(0);
1301   Value *FirstOffset = FirstGEP->getOperand(1);
1302   // Give up if the index of the first GEP is loop invariant.
1303   if (CurLoop->isLoopInvariant(FirstOffset))
1304     return false;
1305 
1306   // Give up if base doesn't have same type.
1307   if (FirstBase->getType() != SecondBase->getType())
1308     return false;
1309 
1310   Instruction *FirstOffsetDef = dyn_cast<Instruction>(FirstOffset);
1311 
1312   // Check if the second operand of first GEP has constant coefficient.
1313   // For an example, for the following code,  we won't gain anything by
1314   // hoisting the second GEP out because the second GEP can be folded away.
1315   //   %scevgep.sum.ur159 = add i64 %idxprom48.ur, 256
1316   //   %67 = shl i64 %scevgep.sum.ur159, 2
1317   //   %uglygep160 = getelementptr i8* %65, i64 %67
1318   //   %uglygep161 = getelementptr i8* %uglygep160, i64 -1024
1319 
1320   // Skip constant shift instruction which may be generated by Splitting GEPs.
1321   if (FirstOffsetDef && FirstOffsetDef->isShift() &&
1322       isa<ConstantInt>(FirstOffsetDef->getOperand(1)))
1323     FirstOffsetDef = dyn_cast<Instruction>(FirstOffsetDef->getOperand(0));
1324 
1325   // Give up if FirstOffsetDef is an Add or Sub with constant.
1326   // Because it may not profitable at all due to constant folding.
1327   if (FirstOffsetDef)
1328     if (BinaryOperator *BO = dyn_cast<BinaryOperator>(FirstOffsetDef)) {
1329       unsigned opc = BO->getOpcode();
1330       if ((opc == Instruction::Add || opc == Instruction::Sub) &&
1331           (isa<ConstantInt>(BO->getOperand(0)) ||
1332            isa<ConstantInt>(BO->getOperand(1))))
1333         return false;
1334     }
1335   return true;
1336 }
1337 
1338 bool SeparateConstOffsetFromGEP::hasMoreThanOneUseInLoop(Value *V, Loop *L) {
1339   int UsesInLoop = 0;
1340   for (User *U : V->users()) {
1341     if (Instruction *User = dyn_cast<Instruction>(U))
1342       if (L->contains(User))
1343         if (++UsesInLoop > 1)
1344           return true;
1345   }
1346   return false;
1347 }
1348 
1349 void SeparateConstOffsetFromGEP::swapGEPOperand(GetElementPtrInst *First,
1350                                                 GetElementPtrInst *Second) {
1351   Value *Offset1 = First->getOperand(1);
1352   Value *Offset2 = Second->getOperand(1);
1353   First->setOperand(1, Offset2);
1354   Second->setOperand(1, Offset1);
1355 
1356   // We changed p+o+c to p+c+o, p+c may not be inbound anymore.
1357   const DataLayout &DAL = First->getModule()->getDataLayout();
1358   APInt Offset(DAL.getIndexSizeInBits(
1359                    cast<PointerType>(First->getType())->getAddressSpace()),
1360                0);
1361   Value *NewBase =
1362       First->stripAndAccumulateInBoundsConstantOffsets(DAL, Offset);
1363   uint64_t ObjectSize;
1364   if (!getObjectSize(NewBase, ObjectSize, DAL, TLI) ||
1365      Offset.ugt(ObjectSize)) {
1366     First->setIsInBounds(false);
1367     Second->setIsInBounds(false);
1368   } else
1369     First->setIsInBounds(true);
1370 }
1371 
1372 PreservedAnalyses
1373 SeparateConstOffsetFromGEPPass::run(Function &F, FunctionAnalysisManager &AM) {
1374   auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
1375   auto *SE = &AM.getResult<ScalarEvolutionAnalysis>(F);
1376   auto *LI = &AM.getResult<LoopAnalysis>(F);
1377   auto *TLI = &AM.getResult<TargetLibraryAnalysis>(F);
1378   auto GetTTI = [&AM](Function &F) -> TargetTransformInfo & {
1379     return AM.getResult<TargetIRAnalysis>(F);
1380   };
1381   SeparateConstOffsetFromGEP Impl(DT, SE, LI, TLI, GetTTI, LowerGEP);
1382   if (!Impl.run(F))
1383     return PreservedAnalyses::all();
1384   PreservedAnalyses PA;
1385   PA.preserveSet<CFGAnalyses>();
1386   return PA;
1387 }
1388