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/LLVMContext.h"
28 #include "llvm/IR/Module.h"
29 #include "llvm/IR/PassManager.h"
30 #include "llvm/InitializePasses.h"
31 #include "llvm/Pass.h"
32 #include "llvm/Transforms/Scalar.h"
33 #include "llvm/Transforms/Utils.h"
34 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
35 #include "llvm/Transforms/Utils/Local.h"
36 #include <list>
37 using namespace llvm;
38 
39 #define DEBUG_TYPE "reg2mem"
40 
41 STATISTIC(NumRegsDemoted, "Number of registers demoted");
42 STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted");
43 
44 static bool valueEscapes(const Instruction &Inst) {
45   const BasicBlock *BB = Inst.getParent();
46   for (const User *U : Inst.users()) {
47     const Instruction *UI = cast<Instruction>(U);
48     if (UI->getParent() != BB || isa<PHINode>(UI))
49       return true;
50   }
51   return false;
52 }
53 
54 static 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>(I)) ++I;
65 
66   CastInst *AllocaInsertionPoint = new BitCastInst(
67       Constant::getNullValue(Type::getInt32Ty(F.getContext())),
68       Type::getInt32Ty(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>(I) && I.getParent() == BBEntry) && valueEscapes(I))
75       WorkList.push_front(&I);
76 
77   // Demote escaped instructions
78   NumRegsDemoted += WorkList.size();
79   for (Instruction *I : WorkList)
80     DemoteRegToStack(*I, false, AllocaInsertionPoint);
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(&Phi);
88 
89   // Demote phi nodes
90   NumPhisDemoted += WorkList.size();
91   for (Instruction *I : WorkList)
92     DemotePHIToStack(cast<PHINode>(I), AllocaInsertionPoint);
93 
94   return true;
95 }
96 
97 PreservedAnalyses RegToMemPass::run(Function &F, FunctionAnalysisManager &AM) {
98   auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
99   auto *LI = &AM.getResult<LoopAnalysis>(F);
100   unsigned N = SplitAllCriticalEdges(F, 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 
110 namespace {
111 struct RegToMemLegacy : public FunctionPass {
112   static char ID; // Pass identification, replacement for typeid
113   RegToMemLegacy() : FunctionPass(ID) {
114     initializeRegToMemLegacyPass(*PassRegistry::getPassRegistry());
115   }
116 
117   void getAnalysisUsage(AnalysisUsage &AU) const override {
118     AU.addRequiredID(BreakCriticalEdgesID);
119     AU.addPreservedID(BreakCriticalEdgesID);
120   }
121 
122   bool runOnFunction(Function &F) override {
123     if (F.isDeclaration() || skipFunction(F))
124       return false;
125     return runPass(F);
126   }
127 };
128 } // namespace
129 
130 char RegToMemLegacy::ID = 0;
131 INITIALIZE_PASS_BEGIN(RegToMemLegacy, "reg2mem",
132                       "Demote all values to stack slots", false, false)
133 INITIALIZE_PASS_DEPENDENCY(BreakCriticalEdges)
134 INITIALIZE_PASS_END(RegToMemLegacy, "reg2mem",
135                     "Demote all values to stack slots", false, false)
136 
137 // createDemoteRegisterToMemory - Provide an entry point to create this pass.
138 char &llvm::DemoteRegisterToMemoryID = RegToMemLegacy::ID;
139 FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
140   return new RegToMemLegacy();
141 }
142