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 const BasicBlock *BB = Inst.getParent();
44 for (const User *U : Inst.users()) {
45 const Instruction *UI = cast<Instruction>(U);
46 if (UI->getParent() != BB || isa<PHINode>(UI))
47 return true;
48 }
49 return false;
50 }
51
runPass(Function & F)52 static bool runPass(Function &F) {
53 // Insert all new allocas into entry block.
54 BasicBlock *BBEntry = &F.getEntryBlock();
55 assert(pred_empty(BBEntry) &&
56 "Entry block to function must not have predecessors!");
57
58 // Find first non-alloca instruction and create insertion point. This is
59 // safe if block is well-formed: it always have terminator, otherwise
60 // we'll get and assertion.
61 BasicBlock::iterator I = BBEntry->begin();
62 while (isa<AllocaInst>(I)) ++I;
63
64 CastInst *AllocaInsertionPoint = new BitCastInst(
65 Constant::getNullValue(Type::getInt32Ty(F.getContext())),
66 Type::getInt32Ty(F.getContext()), "reg2mem alloca point", &*I);
67
68 // Find the escaped instructions. But don't create stack slots for
69 // allocas in entry block.
70 std::list<Instruction*> WorkList;
71 for (Instruction &I : instructions(F))
72 if (!(isa<AllocaInst>(I) && I.getParent() == BBEntry) && valueEscapes(I))
73 WorkList.push_front(&I);
74
75 // Demote escaped instructions
76 NumRegsDemoted += WorkList.size();
77 for (Instruction *I : WorkList)
78 DemoteRegToStack(*I, false, AllocaInsertionPoint);
79
80 WorkList.clear();
81
82 // Find all phi's
83 for (BasicBlock &BB : F)
84 for (auto &Phi : BB.phis())
85 WorkList.push_front(&Phi);
86
87 // Demote phi nodes
88 NumPhisDemoted += WorkList.size();
89 for (Instruction *I : WorkList)
90 DemotePHIToStack(cast<PHINode>(I), AllocaInsertionPoint);
91
92 return true;
93 }
94
run(Function & F,FunctionAnalysisManager & AM)95 PreservedAnalyses RegToMemPass::run(Function &F, FunctionAnalysisManager &AM) {
96 auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
97 auto *LI = &AM.getResult<LoopAnalysis>(F);
98 unsigned N = SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI));
99 bool Changed = runPass(F);
100 if (N == 0 && !Changed)
101 return PreservedAnalyses::all();
102 PreservedAnalyses PA;
103 PA.preserve<DominatorTreeAnalysis>();
104 PA.preserve<LoopAnalysis>();
105 return PA;
106 }
107
108 namespace {
109 struct RegToMemLegacy : public FunctionPass {
110 static char ID; // Pass identification, replacement for typeid
RegToMemLegacy__anone2a257f20111::RegToMemLegacy111 RegToMemLegacy() : FunctionPass(ID) {
112 initializeRegToMemLegacyPass(*PassRegistry::getPassRegistry());
113 }
114
getAnalysisUsage__anone2a257f20111::RegToMemLegacy115 void getAnalysisUsage(AnalysisUsage &AU) const override {
116 AU.addRequiredID(BreakCriticalEdgesID);
117 AU.addPreservedID(BreakCriticalEdgesID);
118 }
119
runOnFunction__anone2a257f20111::RegToMemLegacy120 bool runOnFunction(Function &F) override {
121 if (F.isDeclaration() || skipFunction(F))
122 return false;
123 return runPass(F);
124 }
125 };
126 } // namespace
127
128 char RegToMemLegacy::ID = 0;
129 INITIALIZE_PASS_BEGIN(RegToMemLegacy, "reg2mem",
130 "Demote all values to stack slots", false, false)
131 INITIALIZE_PASS_DEPENDENCY(BreakCriticalEdges)
132 INITIALIZE_PASS_END(RegToMemLegacy, "reg2mem",
133 "Demote all values to stack slots", false, false)
134
135 // createDemoteRegisterToMemory - Provide an entry point to create this pass.
136 char &llvm::DemoteRegisterToMemoryID = RegToMemLegacy::ID;
createDemoteRegisterToMemoryPass()137 FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
138 return new RegToMemLegacy();
139 }
140