1 //===- MakeGuardsExplicit.cpp - Turn guard intrinsics into guard branches -===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This pass lowers the @llvm.experimental.guard intrinsic to the new form of
11 // guard represented as widenable explicit branch to the deopt block. The
12 // difference between this pass and LowerGuardIntrinsic is that after this pass
13 // the guard represented as intrinsic:
14 //
15 //   call void(i1, ...) @llvm.experimental.guard(i1 %old_cond) [ "deopt"() ]
16 //
17 // transforms to a guard represented as widenable explicit branch:
18 //
19 //   %widenable_cond = call i1 @llvm.experimental.widenable.condition()
20 //   br i1 (%old_cond & %widenable_cond), label %guarded, label %deopt
21 //
22 // Here:
23 //   - The semantics of @llvm.experimental.widenable.condition allows to replace
24 //     %widenable_cond with the construction (%widenable_cond & %any_other_cond)
25 //     without loss of correctness;
26 //   - %guarded is the lower part of old guard intrinsic's parent block split by
27 //     the intrinsic call;
28 //   - %deopt is a block containing a sole call to @llvm.experimental.deoptimize
29 //     intrinsic.
30 //
31 // Therefore, this branch preserves the property of widenability.
32 //
33 //===----------------------------------------------------------------------===//
34 
35 #include "llvm/Transforms/Scalar/MakeGuardsExplicit.h"
36 #include "llvm/Analysis/GuardUtils.h"
37 #include "llvm/IR/InstIterator.h"
38 #include "llvm/IR/IntrinsicInst.h"
39 #include "llvm/IR/Intrinsics.h"
40 #include "llvm/IR/IRBuilder.h"
41 #include "llvm/Pass.h"
42 #include "llvm/Transforms/Scalar.h"
43 #include "llvm/Transforms/Utils/GuardUtils.h"
44 
45 using namespace llvm;
46 
47 namespace {
48 struct MakeGuardsExplicitLegacyPass : public FunctionPass {
49   static char ID;
50   MakeGuardsExplicitLegacyPass() : FunctionPass(ID) {
51     initializeMakeGuardsExplicitLegacyPassPass(*PassRegistry::getPassRegistry());
52   }
53 
54   bool runOnFunction(Function &F) override;
55 };
56 }
57 
58 static void turnToExplicitForm(CallInst *Guard, Function *DeoptIntrinsic) {
59   // Replace the guard with an explicit branch (just like in GuardWidening).
60   BasicBlock *BB = Guard->getParent();
61   makeGuardControlFlowExplicit(DeoptIntrinsic, Guard);
62   BranchInst *ExplicitGuard = cast<BranchInst>(BB->getTerminator());
63   assert(ExplicitGuard->isConditional() && "Must be!");
64 
65   // We want the guard to be expressed as explicit control flow, but still be
66   // widenable. For that, we add Widenable Condition intrinsic call to the
67   // guard's condition.
68   IRBuilder<> B(ExplicitGuard);
69   auto *WidenableCondition =
70       B.CreateIntrinsic(Intrinsic::experimental_widenable_condition,
71                         {}, {}, nullptr, "widenable_cond");
72   WidenableCondition->setCallingConv(Guard->getCallingConv());
73   auto *NewCond =
74       B.CreateAnd(ExplicitGuard->getCondition(), WidenableCondition);
75   NewCond->setName("exiplicit_guard_cond");
76   ExplicitGuard->setCondition(NewCond);
77   Guard->eraseFromParent();
78 }
79 
80 static bool explicifyGuards(Function &F) {
81   // Check if we can cheaply rule out the possibility of not having any work to
82   // do.
83   auto *GuardDecl = F.getParent()->getFunction(
84       Intrinsic::getName(Intrinsic::experimental_guard));
85   if (!GuardDecl || GuardDecl->use_empty())
86     return false;
87 
88   SmallVector<CallInst *, 8> GuardIntrinsics;
89   for (auto &I : instructions(F))
90     if (isGuard(&I))
91       GuardIntrinsics.push_back(cast<CallInst>(&I));
92 
93   if (GuardIntrinsics.empty())
94     return false;
95 
96   auto *DeoptIntrinsic = Intrinsic::getDeclaration(
97       F.getParent(), Intrinsic::experimental_deoptimize, {F.getReturnType()});
98   DeoptIntrinsic->setCallingConv(GuardDecl->getCallingConv());
99 
100   for (auto *Guard : GuardIntrinsics)
101     turnToExplicitForm(Guard, DeoptIntrinsic);
102 
103   return true;
104 }
105 
106 bool MakeGuardsExplicitLegacyPass::runOnFunction(Function &F) {
107   return explicifyGuards(F);
108 }
109 
110 char MakeGuardsExplicitLegacyPass::ID = 0;
111 INITIALIZE_PASS(MakeGuardsExplicitLegacyPass, "make-guards-explicit",
112                 "Lower the guard intrinsic to explicit control flow form",
113                 false, false)
114 
115 PreservedAnalyses MakeGuardsExplicitPass::run(Function &F,
116                                            FunctionAnalysisManager &) {
117   if (explicifyGuards(F))
118     return PreservedAnalyses::none();
119   return PreservedAnalyses::all();
120 }
121