1 //===- SCCP.cpp - Sparse Conditional Constant Propagation -----------------===//
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 file implements sparse conditional constant propagation and merging:
10 //
11 // Specifically, this:
12 // * Assumes values are constant unless proven otherwise
13 // * Assumes BasicBlocks are dead unless proven otherwise
14 // * Proves values to be constant, and replaces them with constants
15 // * Proves conditional branches to be unconditional
16 //
17 //===----------------------------------------------------------------------===//
18
19 #include "llvm/Transforms/Scalar/SCCP.h"
20 #include "llvm/ADT/STLExtras.h"
21 #include "llvm/ADT/SmallPtrSet.h"
22 #include "llvm/ADT/SmallVector.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/Analysis/DomTreeUpdater.h"
25 #include "llvm/Analysis/GlobalsModRef.h"
26 #include "llvm/Analysis/TargetLibraryInfo.h"
27 #include "llvm/Analysis/ValueTracking.h"
28 #include "llvm/IR/BasicBlock.h"
29 #include "llvm/IR/Constant.h"
30 #include "llvm/IR/DerivedTypes.h"
31 #include "llvm/IR/Function.h"
32 #include "llvm/IR/GlobalVariable.h"
33 #include "llvm/IR/InstrTypes.h"
34 #include "llvm/IR/Instruction.h"
35 #include "llvm/IR/Instructions.h"
36 #include "llvm/IR/Module.h"
37 #include "llvm/IR/PassManager.h"
38 #include "llvm/IR/Type.h"
39 #include "llvm/IR/User.h"
40 #include "llvm/IR/Value.h"
41 #include "llvm/Pass.h"
42 #include "llvm/Support/Casting.h"
43 #include "llvm/Support/Debug.h"
44 #include "llvm/Support/ErrorHandling.h"
45 #include "llvm/Support/raw_ostream.h"
46 #include "llvm/Transforms/Scalar.h"
47 #include "llvm/Transforms/Utils/Local.h"
48 #include "llvm/Transforms/Utils/SCCPSolver.h"
49 #include <utility>
50
51 using namespace llvm;
52
53 #define DEBUG_TYPE "sccp"
54
55 STATISTIC(NumInstRemoved, "Number of instructions removed");
56 STATISTIC(NumDeadBlocks , "Number of basic blocks unreachable");
57 STATISTIC(NumInstReplaced,
58 "Number of instructions replaced with (simpler) instruction");
59
60 // runSCCP() - Run the Sparse Conditional Constant Propagation algorithm,
61 // and return true if the function was modified.
runSCCP(Function & F,const DataLayout & DL,const TargetLibraryInfo * TLI,DomTreeUpdater & DTU)62 static bool runSCCP(Function &F, const DataLayout &DL,
63 const TargetLibraryInfo *TLI, DomTreeUpdater &DTU) {
64 LLVM_DEBUG(dbgs() << "SCCP on function '" << F.getName() << "'\n");
65 SCCPSolver Solver(
66 DL, [TLI](Function &F) -> const TargetLibraryInfo & { return *TLI; },
67 F.getContext());
68
69 // Mark the first block of the function as being executable.
70 Solver.markBlockExecutable(&F.front());
71
72 // Mark all arguments to the function as being overdefined.
73 for (Argument &AI : F.args())
74 Solver.markOverdefined(&AI);
75
76 // Solve for constants.
77 bool ResolvedUndefs = true;
78 while (ResolvedUndefs) {
79 Solver.solve();
80 LLVM_DEBUG(dbgs() << "RESOLVING UNDEFs\n");
81 ResolvedUndefs = Solver.resolvedUndefsIn(F);
82 }
83
84 bool MadeChanges = false;
85
86 // If we decided that there are basic blocks that are dead in this function,
87 // delete their contents now. Note that we cannot actually delete the blocks,
88 // as we cannot modify the CFG of the function.
89
90 SmallPtrSet<Value *, 32> InsertedValues;
91 SmallVector<BasicBlock *, 8> BlocksToErase;
92 for (BasicBlock &BB : F) {
93 if (!Solver.isBlockExecutable(&BB)) {
94 LLVM_DEBUG(dbgs() << " BasicBlock Dead:" << BB);
95 ++NumDeadBlocks;
96 BlocksToErase.push_back(&BB);
97 MadeChanges = true;
98 continue;
99 }
100
101 MadeChanges |= Solver.simplifyInstsInBlock(BB, InsertedValues,
102 NumInstRemoved, NumInstReplaced);
103 }
104
105 // Remove unreachable blocks and non-feasible edges.
106 for (BasicBlock *DeadBB : BlocksToErase)
107 NumInstRemoved += changeToUnreachable(DeadBB->getFirstNonPHI(),
108 /*PreserveLCSSA=*/false, &DTU);
109
110 BasicBlock *NewUnreachableBB = nullptr;
111 for (BasicBlock &BB : F)
112 MadeChanges |= Solver.removeNonFeasibleEdges(&BB, DTU, NewUnreachableBB);
113
114 for (BasicBlock *DeadBB : BlocksToErase)
115 if (!DeadBB->hasAddressTaken())
116 DTU.deleteBB(DeadBB);
117
118 return MadeChanges;
119 }
120
run(Function & F,FunctionAnalysisManager & AM)121 PreservedAnalyses SCCPPass::run(Function &F, FunctionAnalysisManager &AM) {
122 const DataLayout &DL = F.getDataLayout();
123 auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
124 auto *DT = AM.getCachedResult<DominatorTreeAnalysis>(F);
125 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
126 if (!runSCCP(F, DL, &TLI, DTU))
127 return PreservedAnalyses::all();
128
129 auto PA = PreservedAnalyses();
130 PA.preserve<DominatorTreeAnalysis>();
131 return PA;
132 }
133