1 //===- Reg2Mem.cpp - Convert registers to allocas -------------------------===//
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 demotes all registers to memory references. It is intended to be
10 // the inverse of PromoteMemoryToRegister. By converting to loads, the only
11 // values live across basic blocks are allocas and loads before phi nodes.
12 // It is intended that this should make CFG hacking much easier.
13 // To make later hacking easier, the entry block is split into two, such that
14 // all introduced allocas and nothing else are in the entry block.
15 //
16 //===----------------------------------------------------------------------===//
17
18 #include "llvm/Transforms/Scalar/Reg2Mem.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Analysis/LoopInfo.h"
21 #include "llvm/IR/BasicBlock.h"
22 #include "llvm/IR/CFG.h"
23 #include "llvm/IR/Dominators.h"
24 #include "llvm/IR/Function.h"
25 #include "llvm/IR/InstIterator.h"
26 #include "llvm/IR/Instructions.h"
27 #include "llvm/IR/PassManager.h"
28 #include "llvm/InitializePasses.h"
29 #include "llvm/Pass.h"
30 #include "llvm/Transforms/Scalar.h"
31 #include "llvm/Transforms/Utils.h"
32 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
33 #include "llvm/Transforms/Utils/Local.h"
34 #include <list>
35 using namespace llvm;
36
37 #define DEBUG_TYPE "reg2mem"
38
39 STATISTIC(NumRegsDemoted, "Number of registers demoted");
40 STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted");
41
valueEscapes(const Instruction & Inst)42 static bool valueEscapes(const Instruction &Inst) {
43 if (!Inst.getType()->isSized())
44 return false;
45
46 const BasicBlock *BB = Inst.getParent();
47 for (const User *U : Inst.users()) {
48 const Instruction *UI = cast<Instruction>(U);
49 if (UI->getParent() != BB || isa<PHINode>(UI))
50 return true;
51 }
52 return false;
53 }
54
runPass(Function & F)55 static bool runPass(Function &F) {
56 // Insert all new allocas into entry block.
57 BasicBlock *BBEntry = &F.getEntryBlock();
58 assert(pred_empty(BBEntry) &&
59 "Entry block to function must not have predecessors!");
60
61 // Find first non-alloca instruction and create insertion point. This is
62 // safe if block is well-formed: it always have terminator, otherwise
63 // we'll get and assertion.
64 BasicBlock::iterator I = BBEntry->begin();
65 while (isa<AllocaInst>(I)) ++I;
66
67 CastInst *AllocaInsertionPoint = new BitCastInst(
68 Constant::getNullValue(Type::getInt32Ty(F.getContext())),
69 Type::getInt32Ty(F.getContext()), "reg2mem alloca point", &*I);
70
71 // Find the escaped instructions. But don't create stack slots for
72 // allocas in entry block.
73 std::list<Instruction*> WorkList;
74 for (Instruction &I : instructions(F))
75 if (!(isa<AllocaInst>(I) && I.getParent() == BBEntry) && valueEscapes(I))
76 WorkList.push_front(&I);
77
78 // Demote escaped instructions
79 NumRegsDemoted += WorkList.size();
80 for (Instruction *I : WorkList)
81 DemoteRegToStack(*I, false, AllocaInsertionPoint);
82
83 WorkList.clear();
84
85 // Find all phi's
86 for (BasicBlock &BB : F)
87 for (auto &Phi : BB.phis())
88 WorkList.push_front(&Phi);
89
90 // Demote phi nodes
91 NumPhisDemoted += WorkList.size();
92 for (Instruction *I : WorkList)
93 DemotePHIToStack(cast<PHINode>(I), AllocaInsertionPoint);
94
95 return true;
96 }
97
run(Function & F,FunctionAnalysisManager & AM)98 PreservedAnalyses RegToMemPass::run(Function &F, FunctionAnalysisManager &AM) {
99 auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
100 auto *LI = &AM.getResult<LoopAnalysis>(F);
101 unsigned N = SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI));
102 bool Changed = runPass(F);
103 if (N == 0 && !Changed)
104 return PreservedAnalyses::all();
105 PreservedAnalyses PA;
106 PA.preserve<DominatorTreeAnalysis>();
107 PA.preserve<LoopAnalysis>();
108 return PA;
109 }
110
111 namespace {
112 struct RegToMemLegacy : public FunctionPass {
113 static char ID; // Pass identification, replacement for typeid
RegToMemLegacy__anonfaae73b90111::RegToMemLegacy114 RegToMemLegacy() : FunctionPass(ID) {
115 initializeRegToMemLegacyPass(*PassRegistry::getPassRegistry());
116 }
117
getAnalysisUsage__anonfaae73b90111::RegToMemLegacy118 void getAnalysisUsage(AnalysisUsage &AU) const override {
119 AU.addRequiredID(BreakCriticalEdgesID);
120 AU.addPreservedID(BreakCriticalEdgesID);
121 }
122
runOnFunction__anonfaae73b90111::RegToMemLegacy123 bool runOnFunction(Function &F) override {
124 if (F.isDeclaration() || skipFunction(F))
125 return false;
126 return runPass(F);
127 }
128 };
129 } // namespace
130
131 char RegToMemLegacy::ID = 0;
132 INITIALIZE_PASS_BEGIN(RegToMemLegacy, "reg2mem",
133 "Demote all values to stack slots", false, false)
134 INITIALIZE_PASS_DEPENDENCY(BreakCriticalEdges)
135 INITIALIZE_PASS_END(RegToMemLegacy, "reg2mem",
136 "Demote all values to stack slots", false, false)
137
138 // createDemoteRegisterToMemory - Provide an entry point to create this pass.
139 char &llvm::DemoteRegisterToMemoryID = RegToMemLegacy::ID;
createDemoteRegisterToMemoryPass()140 FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
141 return new RegToMemLegacy();
142 }
143