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 
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 
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 
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
114   RegToMemLegacy() : FunctionPass(ID) {
115     initializeRegToMemLegacyPass(*PassRegistry::getPassRegistry());
116   }
117 
118   void getAnalysisUsage(AnalysisUsage &AU) const override {
119     AU.addRequiredID(BreakCriticalEdgesID);
120     AU.addPreservedID(BreakCriticalEdgesID);
121   }
122 
123   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;
140 FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
141   return new RegToMemLegacy();
142 }
143