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/Transforms/Scalar.h"
30#include "llvm/Transforms/Utils.h"
31#include "llvm/Transforms/Utils/BasicBlockUtils.h"
32#include "llvm/Transforms/Utils/Local.h"
33#include <list>
34using namespace llvm;
35
36#define DEBUG_TYPE "reg2mem"
37
38STATISTIC(NumRegsDemoted, "Number of registers demoted");
39STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted");
40
41static bool valueEscapes(const Instruction &Inst) {
42 if (!Inst.getType()->isSized())
43 return false;
44
45 const BasicBlock *BB = Inst.getParent();
46 for (const User *U : Inst.users()) {
47 const Instruction *UI = cast<Instruction>(Val: U);
48 if (UI->getParent() != BB || isa<PHINode>(Val: UI))
49 return true;
50 }
51 return false;
52}
53
54static bool runPass(Function &F) {
55 // Insert all new allocas into entry block.
56 BasicBlock *BBEntry = &F.getEntryBlock();
57 assert(pred_empty(BBEntry) &&
58 "Entry block to function must not have predecessors!");
59
60 // Find first non-alloca instruction and create insertion point. This is
61 // safe if block is well-formed: it always have terminator, otherwise
62 // we'll get and assertion.
63 BasicBlock::iterator I = BBEntry->begin();
64 while (isa<AllocaInst>(Val: I)) ++I;
65
66 CastInst *AllocaInsertionPoint = new BitCastInst(
67 Constant::getNullValue(Ty: Type::getInt32Ty(C&: F.getContext())),
68 Type::getInt32Ty(C&: F.getContext()), "reg2mem alloca point", I);
69
70 // Find the escaped instructions. But don't create stack slots for
71 // allocas in entry block.
72 std::list<Instruction*> WorkList;
73 for (Instruction &I : instructions(F))
74 if (!(isa<AllocaInst>(Val: I) && I.getParent() == BBEntry) && valueEscapes(Inst: I))
75 WorkList.push_front(x: &I);
76
77 // Demote escaped instructions
78 NumRegsDemoted += WorkList.size();
79 for (Instruction *I : WorkList)
80 DemoteRegToStack(X&: *I, VolatileLoads: false, AllocaPoint: AllocaInsertionPoint->getIterator());
81
82 WorkList.clear();
83
84 // Find all phi's
85 for (BasicBlock &BB : F)
86 for (auto &Phi : BB.phis())
87 WorkList.push_front(x: &Phi);
88
89 // Demote phi nodes
90 NumPhisDemoted += WorkList.size();
91 for (Instruction *I : WorkList)
92 DemotePHIToStack(P: cast<PHINode>(Val: I), AllocaPoint: AllocaInsertionPoint->getIterator());
93
94 return true;
95}
96
97PreservedAnalyses RegToMemPass::run(Function &F, FunctionAnalysisManager &AM) {
98 auto *DT = &AM.getResult<DominatorTreeAnalysis>(IR&: F);
99 auto *LI = &AM.getResult<LoopAnalysis>(IR&: F);
100 unsigned N = SplitAllCriticalEdges(F, Options: CriticalEdgeSplittingOptions(DT, LI));
101 bool Changed = runPass(F);
102 if (N == 0 && !Changed)
103 return PreservedAnalyses::all();
104 PreservedAnalyses PA;
105 PA.preserve<DominatorTreeAnalysis>();
106 PA.preserve<LoopAnalysis>();
107 return PA;
108}
109
110namespace llvm {
111
112void initializeRegToMemWrapperPassPass(PassRegistry &);
113
114class RegToMemWrapperPass : public FunctionPass {
115public:
116 static char ID;
117
118 RegToMemWrapperPass() : FunctionPass(ID) {}
119
120 void getAnalysisUsage(AnalysisUsage &AU) const override {
121 AU.setPreservesAll();
122
123 AU.addPreserved<DominatorTreeWrapperPass>();
124 AU.addRequired<DominatorTreeWrapperPass>();
125
126 AU.addPreserved<LoopInfoWrapperPass>();
127 AU.addRequired<LoopInfoWrapperPass>();
128 }
129
130 bool runOnFunction(Function &F) override {
131 DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
132 LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
133
134 unsigned N = SplitAllCriticalEdges(F, Options: CriticalEdgeSplittingOptions(DT, LI));
135 bool Changed = runPass(F);
136 return N != 0 || Changed;
137 }
138};
139} // namespace llvm
140
141INITIALIZE_PASS_BEGIN(RegToMemWrapperPass, "reg2mem", "", true, true)
142INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass);
143INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass);
144INITIALIZE_PASS_END(RegToMemWrapperPass, "reg2mem", "", true, true)
145
146char RegToMemWrapperPass::ID = 0;
147
148FunctionPass *llvm::createRegToMemWrapperPass() {
149 return new RegToMemWrapperPass();
150}
151