lib/Transforms/Scalar/MakeGuardsExplicit.cpp

   1 //===- MakeGuardsExplicit.cpp - Turn guard intrinsics into guard branches -===//
   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 pass lowers the @llvm.experimental.guard intrinsic to the new form of
  10 // guard represented as widenable explicit branch to the deopt block. The
  11 // difference between this pass and LowerGuardIntrinsic is that after this pass
  12 // the guard represented as intrinsic:
  13 //
  14 //   call void(i1, ...) @llvm.experimental.guard(i1 %old_cond) [ "deopt"() ]
  15 //
  16 // transforms to a guard represented as widenable explicit branch:
  17 //
  18 //   %widenable_cond = call i1 @llvm.experimental.widenable.condition()
  19 //   br i1 (%old_cond & %widenable_cond), label %guarded, label %deopt
  20 //
  21 // Here:
  22 //   - The semantics of @llvm.experimental.widenable.condition allows to replace
  23 //     %widenable_cond with the construction (%widenable_cond & %any_other_cond)
  24 //     without loss of correctness;
  25 //   - %guarded is the lower part of old guard intrinsic's parent block split by
  26 //     the intrinsic call;
  27 //   - %deopt is a block containing a sole call to @llvm.experimental.deoptimize
  28 //     intrinsic.
  29 //
  30 // Therefore, this branch preserves the property of widenability.
  31 //
  32 //===----------------------------------------------------------------------===//
  33
  34 #include "llvm/Transforms/Scalar/MakeGuardsExplicit.h"
  35 #include "llvm/Analysis/GuardUtils.h"
  36 #include "llvm/IR/InstIterator.h"
  37 #include "llvm/IR/IntrinsicInst.h"
  38 #include "llvm/IR/Intrinsics.h"
  39 #include "llvm/IR/IRBuilder.h"
  40 #include "llvm/Pass.h"
  41 #include "llvm/Transforms/Scalar.h"
  42 #include "llvm/Transforms/Utils/GuardUtils.h"
  43
  44 using namespace llvm;
  45
  46 namespace {
  47 struct MakeGuardsExplicitLegacyPass : public FunctionPass {
  48   static char ID;
  49   MakeGuardsExplicitLegacyPass() : FunctionPass(ID) {
  50     initializeMakeGuardsExplicitLegacyPassPass(*PassRegistry::getPassRegistry());
  51   }
  52
  53   bool runOnFunction(Function &F) override;
  54 };
  55 }
  56
  57 static void turnToExplicitForm(CallInst *Guard, Function *DeoptIntrinsic) {
  58   // Replace the guard with an explicit branch (just like in GuardWidening).
  59   BasicBlock *BB = Guard->getParent();
  60   makeGuardControlFlowExplicit(DeoptIntrinsic, Guard);
  61   BranchInst *ExplicitGuard = cast<BranchInst>(BB->getTerminator());
  62   assert(ExplicitGuard->isConditional() && "Must be!");
  63
  64   // We want the guard to be expressed as explicit control flow, but still be
  65   // widenable. For that, we add Widenable Condition intrinsic call to the
  66   // guard's condition.
  67   IRBuilder<> B(ExplicitGuard);
  68   auto *WidenableCondition =
  69       B.CreateIntrinsic(Intrinsic::experimental_widenable_condition,
  70                         {}, {}, nullptr, "widenable_cond");
  71   WidenableCondition->setCallingConv(Guard->getCallingConv());
  72   auto *NewCond =
  73       B.CreateAnd(ExplicitGuard->getCondition(), WidenableCondition);
  74   NewCond->setName("exiplicit_guard_cond");
  75   ExplicitGuard->setCondition(NewCond);
  76   Guard->eraseFromParent();
  77 }
  78
  79 static bool explicifyGuards(Function &F) {
  80   // Check if we can cheaply rule out the possibility of not having any work to
  81   // do.
  82   auto *GuardDecl = F.getParent()->getFunction(
  83       Intrinsic::getName(Intrinsic::experimental_guard));
  84   if (!GuardDecl || GuardDecl->use_empty())
  85     return false;
  86
  87   SmallVector<CallInst *, 8> GuardIntrinsics;
  88   for (auto &I : instructions(F))
  89     if (isGuard(&I))
  90       GuardIntrinsics.push_back(cast<CallInst>(&I));
  91
  92   if (GuardIntrinsics.empty())
  93     return false;
  94
  95   auto *DeoptIntrinsic = Intrinsic::getDeclaration(
  96       F.getParent(), Intrinsic::experimental_deoptimize, {F.getReturnType()});
  97   DeoptIntrinsic->setCallingConv(GuardDecl->getCallingConv());
  98
  99   for (auto *Guard : GuardIntrinsics)
 100     turnToExplicitForm(Guard, DeoptIntrinsic);
 101
 102   return true;
 103 }
 104
 105 bool MakeGuardsExplicitLegacyPass::runOnFunction(Function &F) {
 106   return explicifyGuards(F);
 107 }
 108
 109 char MakeGuardsExplicitLegacyPass::ID = 0;
 110 INITIALIZE_PASS(MakeGuardsExplicitLegacyPass, "make-guards-explicit",
 111                 "Lower the guard intrinsic to explicit control flow form",
 112                 false, false)
 113
 114 PreservedAnalyses MakeGuardsExplicitPass::run(Function &F,
 115                                            FunctionAnalysisManager &) {
 116   if (explicifyGuards(F))
 117     return PreservedAnalyses::none();
 118   return PreservedAnalyses::all();
 119 }