xref: /freebsd/contrib/llvm-project/llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp (revision ec0ea6efa1ad229d75c394c1a9b9cac33af2b1d3)
1 //===- LowerExpectIntrinsic.cpp - Lower expect intrinsic ------------------===//
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 // This pass lowers the 'expect' intrinsic to LLVM metadata.
10 //
11 //===----------------------------------------------------------------------===//
12 
13 #include "llvm/Transforms/Scalar/LowerExpectIntrinsic.h"
14 #include "llvm/ADT/SmallVector.h"
15 #include "llvm/ADT/Statistic.h"
16 #include "llvm/ADT/iterator_range.h"
17 #include "llvm/IR/BasicBlock.h"
18 #include "llvm/IR/Constants.h"
19 #include "llvm/IR/Function.h"
20 #include "llvm/IR/Instructions.h"
21 #include "llvm/IR/Intrinsics.h"
22 #include "llvm/IR/LLVMContext.h"
23 #include "llvm/IR/MDBuilder.h"
24 #include "llvm/IR/Metadata.h"
25 #include "llvm/InitializePasses.h"
26 #include "llvm/Pass.h"
27 #include "llvm/Support/CommandLine.h"
28 #include "llvm/Support/Debug.h"
29 #include "llvm/Transforms/Scalar.h"
30 
31 using namespace llvm;
32 
33 #define DEBUG_TYPE "lower-expect-intrinsic"
34 
35 STATISTIC(ExpectIntrinsicsHandled,
36           "Number of 'expect' intrinsic instructions handled");
37 
38 // These default values are chosen to represent an extremely skewed outcome for
39 // a condition, but they leave some room for interpretation by later passes.
40 //
41 // If the documentation for __builtin_expect() was made explicit that it should
42 // only be used in extreme cases, we could make this ratio higher. As it stands,
43 // programmers may be using __builtin_expect() / llvm.expect to annotate that a
44 // branch is likely or unlikely to be taken.
45 
46 // WARNING: these values are internal implementation detail of the pass.
47 // They should not be exposed to the outside of the pass, front-end codegen
48 // should emit @llvm.expect intrinsics instead of using these weights directly.
49 // Transforms should use TargetTransformInfo's getPredictableBranchThreshold().
50 static cl::opt<uint32_t> LikelyBranchWeight(
51     "likely-branch-weight", cl::Hidden, cl::init(2000),
52     cl::desc("Weight of the branch likely to be taken (default = 2000)"));
53 static cl::opt<uint32_t> UnlikelyBranchWeight(
54     "unlikely-branch-weight", cl::Hidden, cl::init(1),
55     cl::desc("Weight of the branch unlikely to be taken (default = 1)"));
56 
57 static std::tuple<uint32_t, uint32_t>
58 getBranchWeight(Intrinsic::ID IntrinsicID, CallInst *CI, int BranchCount) {
59   if (IntrinsicID == Intrinsic::expect) {
60     // __builtin_expect
61     return std::make_tuple(LikelyBranchWeight.getValue(),
62                            UnlikelyBranchWeight.getValue());
63   } else {
64     // __builtin_expect_with_probability
65     assert(CI->getNumOperands() >= 3 &&
66            "expect with probability must have 3 arguments");
67     ConstantFP *Confidence = dyn_cast<ConstantFP>(CI->getArgOperand(2));
68     double TrueProb = Confidence->getValueAPF().convertToDouble();
69     assert((TrueProb >= 0.0 && TrueProb <= 1.0) &&
70            "probability value must be in the range [0.0, 1.0]");
71     double FalseProb = (1.0 - TrueProb) / (BranchCount - 1);
72     uint32_t LikelyBW = ceil((TrueProb * (double)(INT32_MAX - 1)) + 1.0);
73     uint32_t UnlikelyBW = ceil((FalseProb * (double)(INT32_MAX - 1)) + 1.0);
74     return std::make_tuple(LikelyBW, UnlikelyBW);
75   }
76 }
77 
78 static bool handleSwitchExpect(SwitchInst &SI) {
79   CallInst *CI = dyn_cast<CallInst>(SI.getCondition());
80   if (!CI)
81     return false;
82 
83   Function *Fn = CI->getCalledFunction();
84   if (!Fn || (Fn->getIntrinsicID() != Intrinsic::expect &&
85               Fn->getIntrinsicID() != Intrinsic::expect_with_probability))
86     return false;
87 
88   Value *ArgValue = CI->getArgOperand(0);
89   ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
90   if (!ExpectedValue)
91     return false;
92 
93   SwitchInst::CaseHandle Case = *SI.findCaseValue(ExpectedValue);
94   unsigned n = SI.getNumCases(); // +1 for default case.
95   uint32_t LikelyBranchWeightVal, UnlikelyBranchWeightVal;
96   std::tie(LikelyBranchWeightVal, UnlikelyBranchWeightVal) =
97       getBranchWeight(Fn->getIntrinsicID(), CI, n + 1);
98 
99   SmallVector<uint32_t, 16> Weights(n + 1, UnlikelyBranchWeightVal);
100 
101   uint64_t Index = (Case == *SI.case_default()) ? 0 : Case.getCaseIndex() + 1;
102   Weights[Index] = LikelyBranchWeightVal;
103 
104   SI.setCondition(ArgValue);
105 
106   SI.setMetadata(LLVMContext::MD_prof,
107                  MDBuilder(CI->getContext()).createBranchWeights(Weights));
108 
109   return true;
110 }
111 
112 /// Handler for PHINodes that define the value argument to an
113 /// @llvm.expect call.
114 ///
115 /// If the operand of the phi has a constant value and it 'contradicts'
116 /// with the expected value of phi def, then the corresponding incoming
117 /// edge of the phi is unlikely to be taken. Using that information,
118 /// the branch probability info for the originating branch can be inferred.
119 static void handlePhiDef(CallInst *Expect) {
120   Value &Arg = *Expect->getArgOperand(0);
121   ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(Expect->getArgOperand(1));
122   if (!ExpectedValue)
123     return;
124   const APInt &ExpectedPhiValue = ExpectedValue->getValue();
125 
126   // Walk up in backward a list of instructions that
127   // have 'copy' semantics by 'stripping' the copies
128   // until a PHI node or an instruction of unknown kind
129   // is reached. Negation via xor is also handled.
130   //
131   //       C = PHI(...);
132   //       B = C;
133   //       A = B;
134   //       D = __builtin_expect(A, 0);
135   //
136   Value *V = &Arg;
137   SmallVector<Instruction *, 4> Operations;
138   while (!isa<PHINode>(V)) {
139     if (ZExtInst *ZExt = dyn_cast<ZExtInst>(V)) {
140       V = ZExt->getOperand(0);
141       Operations.push_back(ZExt);
142       continue;
143     }
144 
145     if (SExtInst *SExt = dyn_cast<SExtInst>(V)) {
146       V = SExt->getOperand(0);
147       Operations.push_back(SExt);
148       continue;
149     }
150 
151     BinaryOperator *BinOp = dyn_cast<BinaryOperator>(V);
152     if (!BinOp || BinOp->getOpcode() != Instruction::Xor)
153       return;
154 
155     ConstantInt *CInt = dyn_cast<ConstantInt>(BinOp->getOperand(1));
156     if (!CInt)
157       return;
158 
159     V = BinOp->getOperand(0);
160     Operations.push_back(BinOp);
161   }
162 
163   // Executes the recorded operations on input 'Value'.
164   auto ApplyOperations = [&](const APInt &Value) {
165     APInt Result = Value;
166     for (auto Op : llvm::reverse(Operations)) {
167       switch (Op->getOpcode()) {
168       case Instruction::Xor:
169         Result ^= cast<ConstantInt>(Op->getOperand(1))->getValue();
170         break;
171       case Instruction::ZExt:
172         Result = Result.zext(Op->getType()->getIntegerBitWidth());
173         break;
174       case Instruction::SExt:
175         Result = Result.sext(Op->getType()->getIntegerBitWidth());
176         break;
177       default:
178         llvm_unreachable("Unexpected operation");
179       }
180     }
181     return Result;
182   };
183 
184   auto *PhiDef = cast<PHINode>(V);
185 
186   // Get the first dominating conditional branch of the operand
187   // i's incoming block.
188   auto GetDomConditional = [&](unsigned i) -> BranchInst * {
189     BasicBlock *BB = PhiDef->getIncomingBlock(i);
190     BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
191     if (BI && BI->isConditional())
192       return BI;
193     BB = BB->getSinglePredecessor();
194     if (!BB)
195       return nullptr;
196     BI = dyn_cast<BranchInst>(BB->getTerminator());
197     if (!BI || BI->isUnconditional())
198       return nullptr;
199     return BI;
200   };
201 
202   // Now walk through all Phi operands to find phi oprerands with values
203   // conflicting with the expected phi output value. Any such operand
204   // indicates the incoming edge to that operand is unlikely.
205   for (unsigned i = 0, e = PhiDef->getNumIncomingValues(); i != e; ++i) {
206 
207     Value *PhiOpnd = PhiDef->getIncomingValue(i);
208     ConstantInt *CI = dyn_cast<ConstantInt>(PhiOpnd);
209     if (!CI)
210       continue;
211 
212     // Not an interesting case when IsUnlikely is false -- we can not infer
213     // anything useful when the operand value matches the expected phi
214     // output.
215     if (ExpectedPhiValue == ApplyOperations(CI->getValue()))
216       continue;
217 
218     BranchInst *BI = GetDomConditional(i);
219     if (!BI)
220       continue;
221 
222     MDBuilder MDB(PhiDef->getContext());
223 
224     // There are two situations in which an operand of the PhiDef comes
225     // from a given successor of a branch instruction BI.
226     // 1) When the incoming block of the operand is the successor block;
227     // 2) When the incoming block is BI's enclosing block and the
228     // successor is the PhiDef's enclosing block.
229     //
230     // Returns true if the operand which comes from OpndIncomingBB
231     // comes from outgoing edge of BI that leads to Succ block.
232     auto *OpndIncomingBB = PhiDef->getIncomingBlock(i);
233     auto IsOpndComingFromSuccessor = [&](BasicBlock *Succ) {
234       if (OpndIncomingBB == Succ)
235         // If this successor is the incoming block for this
236         // Phi operand, then this successor does lead to the Phi.
237         return true;
238       if (OpndIncomingBB == BI->getParent() && Succ == PhiDef->getParent())
239         // Otherwise, if the edge is directly from the branch
240         // to the Phi, this successor is the one feeding this
241         // Phi operand.
242         return true;
243       return false;
244     };
245     uint32_t LikelyBranchWeightVal, UnlikelyBranchWeightVal;
246     std::tie(LikelyBranchWeightVal, UnlikelyBranchWeightVal) = getBranchWeight(
247         Expect->getCalledFunction()->getIntrinsicID(), Expect, 2);
248 
249     if (IsOpndComingFromSuccessor(BI->getSuccessor(1)))
250       BI->setMetadata(LLVMContext::MD_prof,
251                       MDB.createBranchWeights(LikelyBranchWeightVal,
252                                               UnlikelyBranchWeightVal));
253     else if (IsOpndComingFromSuccessor(BI->getSuccessor(0)))
254       BI->setMetadata(LLVMContext::MD_prof,
255                       MDB.createBranchWeights(UnlikelyBranchWeightVal,
256                                               LikelyBranchWeightVal));
257   }
258 }
259 
260 // Handle both BranchInst and SelectInst.
261 template <class BrSelInst> static bool handleBrSelExpect(BrSelInst &BSI) {
262 
263   // Handle non-optimized IR code like:
264   //   %expval = call i64 @llvm.expect.i64(i64 %conv1, i64 1)
265   //   %tobool = icmp ne i64 %expval, 0
266   //   br i1 %tobool, label %if.then, label %if.end
267   //
268   // Or the following simpler case:
269   //   %expval = call i1 @llvm.expect.i1(i1 %cmp, i1 1)
270   //   br i1 %expval, label %if.then, label %if.end
271 
272   CallInst *CI;
273 
274   ICmpInst *CmpI = dyn_cast<ICmpInst>(BSI.getCondition());
275   CmpInst::Predicate Predicate;
276   ConstantInt *CmpConstOperand = nullptr;
277   if (!CmpI) {
278     CI = dyn_cast<CallInst>(BSI.getCondition());
279     Predicate = CmpInst::ICMP_NE;
280   } else {
281     Predicate = CmpI->getPredicate();
282     if (Predicate != CmpInst::ICMP_NE && Predicate != CmpInst::ICMP_EQ)
283       return false;
284 
285     CmpConstOperand = dyn_cast<ConstantInt>(CmpI->getOperand(1));
286     if (!CmpConstOperand)
287       return false;
288     CI = dyn_cast<CallInst>(CmpI->getOperand(0));
289   }
290 
291   if (!CI)
292     return false;
293 
294   uint64_t ValueComparedTo = 0;
295   if (CmpConstOperand) {
296     if (CmpConstOperand->getBitWidth() > 64)
297       return false;
298     ValueComparedTo = CmpConstOperand->getZExtValue();
299   }
300 
301   Function *Fn = CI->getCalledFunction();
302   if (!Fn || (Fn->getIntrinsicID() != Intrinsic::expect &&
303               Fn->getIntrinsicID() != Intrinsic::expect_with_probability))
304     return false;
305 
306   Value *ArgValue = CI->getArgOperand(0);
307   ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
308   if (!ExpectedValue)
309     return false;
310 
311   MDBuilder MDB(CI->getContext());
312   MDNode *Node;
313 
314   uint32_t LikelyBranchWeightVal, UnlikelyBranchWeightVal;
315   std::tie(LikelyBranchWeightVal, UnlikelyBranchWeightVal) =
316       getBranchWeight(Fn->getIntrinsicID(), CI, 2);
317 
318   if ((ExpectedValue->getZExtValue() == ValueComparedTo) ==
319       (Predicate == CmpInst::ICMP_EQ)) {
320     Node =
321         MDB.createBranchWeights(LikelyBranchWeightVal, UnlikelyBranchWeightVal);
322   } else {
323     Node =
324         MDB.createBranchWeights(UnlikelyBranchWeightVal, LikelyBranchWeightVal);
325   }
326 
327   if (CmpI)
328     CmpI->setOperand(0, ArgValue);
329   else
330     BSI.setCondition(ArgValue);
331 
332   BSI.setMetadata(LLVMContext::MD_prof, Node);
333 
334   return true;
335 }
336 
337 static bool handleBranchExpect(BranchInst &BI) {
338   if (BI.isUnconditional())
339     return false;
340 
341   return handleBrSelExpect<BranchInst>(BI);
342 }
343 
344 static bool lowerExpectIntrinsic(Function &F) {
345   bool Changed = false;
346 
347   for (BasicBlock &BB : F) {
348     // Create "block_weights" metadata.
349     if (BranchInst *BI = dyn_cast<BranchInst>(BB.getTerminator())) {
350       if (handleBranchExpect(*BI))
351         ExpectIntrinsicsHandled++;
352     } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB.getTerminator())) {
353       if (handleSwitchExpect(*SI))
354         ExpectIntrinsicsHandled++;
355     }
356 
357     // Remove llvm.expect intrinsics. Iterate backwards in order
358     // to process select instructions before the intrinsic gets
359     // removed.
360     for (auto BI = BB.rbegin(), BE = BB.rend(); BI != BE;) {
361       Instruction *Inst = &*BI++;
362       CallInst *CI = dyn_cast<CallInst>(Inst);
363       if (!CI) {
364         if (SelectInst *SI = dyn_cast<SelectInst>(Inst)) {
365           if (handleBrSelExpect(*SI))
366             ExpectIntrinsicsHandled++;
367         }
368         continue;
369       }
370 
371       Function *Fn = CI->getCalledFunction();
372       if (Fn && (Fn->getIntrinsicID() == Intrinsic::expect ||
373                  Fn->getIntrinsicID() == Intrinsic::expect_with_probability)) {
374         // Before erasing the llvm.expect, walk backward to find
375         // phi that define llvm.expect's first arg, and
376         // infer branch probability:
377         handlePhiDef(CI);
378         Value *Exp = CI->getArgOperand(0);
379         CI->replaceAllUsesWith(Exp);
380         CI->eraseFromParent();
381         Changed = true;
382       }
383     }
384   }
385 
386   return Changed;
387 }
388 
389 PreservedAnalyses LowerExpectIntrinsicPass::run(Function &F,
390                                                 FunctionAnalysisManager &) {
391   if (lowerExpectIntrinsic(F))
392     return PreservedAnalyses::none();
393 
394   return PreservedAnalyses::all();
395 }
396 
397 namespace {
398 /// Legacy pass for lowering expect intrinsics out of the IR.
399 ///
400 /// When this pass is run over a function it uses expect intrinsics which feed
401 /// branches and switches to provide branch weight metadata for those
402 /// terminators. It then removes the expect intrinsics from the IR so the rest
403 /// of the optimizer can ignore them.
404 class LowerExpectIntrinsic : public FunctionPass {
405 public:
406   static char ID;
407   LowerExpectIntrinsic() : FunctionPass(ID) {
408     initializeLowerExpectIntrinsicPass(*PassRegistry::getPassRegistry());
409   }
410 
411   bool runOnFunction(Function &F) override { return lowerExpectIntrinsic(F); }
412 };
413 }
414 
415 char LowerExpectIntrinsic::ID = 0;
416 INITIALIZE_PASS(LowerExpectIntrinsic, "lower-expect",
417                 "Lower 'expect' Intrinsics", false, false)
418 
419 FunctionPass *llvm::createLowerExpectIntrinsicPass() {
420   return new LowerExpectIntrinsic();
421 }
422