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/Transforms/Scalar.h"
29 #include "llvm/Transforms/Utils.h"
30 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
31 #include "llvm/Transforms/Utils/Local.h"
32 #include <list>
33 using namespace llvm;
34
35 #define DEBUG_TYPE "reg2mem"
36
37 STATISTIC(NumRegsDemoted, "Number of registers demoted");
38 STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted");
39
valueEscapes(const Instruction & Inst)40 static bool valueEscapes(const Instruction &Inst) {
41 if (!Inst.getType()->isSized())
42 return false;
43
44 const BasicBlock *BB = Inst.getParent();
45 for (const User *U : Inst.users()) {
46 const Instruction *UI = cast<Instruction>(U);
47 if (UI->getParent() != BB || isa<PHINode>(UI))
48 return true;
49 }
50 return false;
51 }
52
runPass(Function & F)53 static bool runPass(Function &F) {
54 // Insert all new allocas into entry block.
55 BasicBlock *BBEntry = &F.getEntryBlock();
56 assert(pred_empty(BBEntry) &&
57 "Entry block to function must not have predecessors!");
58
59 // Find first non-alloca instruction and create insertion point. This is
60 // safe if block is well-formed: it always have terminator, otherwise
61 // we'll get and assertion.
62 BasicBlock::iterator I = BBEntry->begin();
63 while (isa<AllocaInst>(I)) ++I;
64
65 CastInst *AllocaInsertionPoint = new BitCastInst(
66 Constant::getNullValue(Type::getInt32Ty(F.getContext())),
67 Type::getInt32Ty(F.getContext()), "reg2mem alloca point", I);
68
69 // Find the escaped instructions. But don't create stack slots for
70 // allocas in entry block.
71 std::list<Instruction*> WorkList;
72 for (Instruction &I : instructions(F))
73 if (!(isa<AllocaInst>(I) && I.getParent() == BBEntry) && valueEscapes(I))
74 WorkList.push_front(&I);
75
76 // Demote escaped instructions
77 NumRegsDemoted += WorkList.size();
78 for (Instruction *I : WorkList)
79 DemoteRegToStack(*I, false, AllocaInsertionPoint->getIterator());
80
81 WorkList.clear();
82
83 // Find all phi's
84 for (BasicBlock &BB : F)
85 for (auto &Phi : BB.phis())
86 WorkList.push_front(&Phi);
87
88 // Demote phi nodes
89 NumPhisDemoted += WorkList.size();
90 for (Instruction *I : WorkList)
91 DemotePHIToStack(cast<PHINode>(I), AllocaInsertionPoint->getIterator());
92
93 return true;
94 }
95
run(Function & F,FunctionAnalysisManager & AM)96 PreservedAnalyses RegToMemPass::run(Function &F, FunctionAnalysisManager &AM) {
97 auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
98 auto *LI = &AM.getResult<LoopAnalysis>(F);
99 unsigned N = SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI));
100 bool Changed = runPass(F);
101 if (N == 0 && !Changed)
102 return PreservedAnalyses::all();
103 PreservedAnalyses PA;
104 PA.preserve<DominatorTreeAnalysis>();
105 PA.preserve<LoopAnalysis>();
106 return PA;
107 }
108