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/Instructions.h"
38 #include "llvm/IR/Intrinsics.h"
39 #include "llvm/InitializePasses.h"
40 #include "llvm/Pass.h"
41 #include "llvm/Transforms/Utils/GuardUtils.h"
42 
43 using namespace llvm;
44 
45 namespace {
46 struct MakeGuardsExplicitLegacyPass : public FunctionPass {
47   static char ID;
48   MakeGuardsExplicitLegacyPass() : FunctionPass(ID) {
49     initializeMakeGuardsExplicitLegacyPassPass(*PassRegistry::getPassRegistry());
50   }
51 
52   bool runOnFunction(Function &F) override;
53 };
54 }
55 
56 static void turnToExplicitForm(CallInst *Guard, Function *DeoptIntrinsic) {
57   // Replace the guard with an explicit branch (just like in GuardWidening).
58   BasicBlock *OriginalBB = Guard->getParent();
59   (void)OriginalBB;
60   makeGuardControlFlowExplicit(DeoptIntrinsic, Guard, true);
61   assert(isWidenableBranch(OriginalBB->getTerminator()) && "should hold");
62 
63   Guard->eraseFromParent();
64 }
65 
66 static bool explicifyGuards(Function &F) {
67   // Check if we can cheaply rule out the possibility of not having any work to
68   // do.
69   auto *GuardDecl = F.getParent()->getFunction(
70       Intrinsic::getName(Intrinsic::experimental_guard));
71   if (!GuardDecl || GuardDecl->use_empty())
72     return false;
73 
74   SmallVector<CallInst *, 8> GuardIntrinsics;
75   for (auto &I : instructions(F))
76     if (isGuard(&I))
77       GuardIntrinsics.push_back(cast<CallInst>(&I));
78 
79   if (GuardIntrinsics.empty())
80     return false;
81 
82   auto *DeoptIntrinsic = Intrinsic::getDeclaration(
83       F.getParent(), Intrinsic::experimental_deoptimize, {F.getReturnType()});
84   DeoptIntrinsic->setCallingConv(GuardDecl->getCallingConv());
85 
86   for (auto *Guard : GuardIntrinsics)
87     turnToExplicitForm(Guard, DeoptIntrinsic);
88 
89   return true;
90 }
91 
92 bool MakeGuardsExplicitLegacyPass::runOnFunction(Function &F) {
93   return explicifyGuards(F);
94 }
95 
96 char MakeGuardsExplicitLegacyPass::ID = 0;
97 INITIALIZE_PASS(MakeGuardsExplicitLegacyPass, "make-guards-explicit",
98                 "Lower the guard intrinsic to explicit control flow form",
99                 false, false)
100 
101 PreservedAnalyses MakeGuardsExplicitPass::run(Function &F,
102                                            FunctionAnalysisManager &) {
103   if (explicifyGuards(F))
104     return PreservedAnalyses::none();
105   return PreservedAnalyses::all();
106 }
107