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Folding compares with unescaped allocations
Summary: If we know that the pointer allocated within a function does not escape, we can fold away comparisons that are done with global pointers Patch by Anna Thomas! Reviewers: reames, majnemer, sanjoy Subscribers: mgrang, mcrosier, majnemer, llvm-commits Differential Revision: http://reviews.llvm.org/D19276 llvm-svn: 267035
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@ -1863,6 +1863,14 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
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return nullptr;
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}
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static bool isNeverEqualToUnescapedAlloc(Value *V) {
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if (isa<ConstantPointerNull>(V))
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return true;
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if (auto *LI = dyn_cast<LoadInst>(V))
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return isa<GlobalVariable>(LI->getPointerOperand());
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return false;
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}
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static bool
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isAllocSiteRemovable(Instruction *AI, SmallVectorImpl<WeakVH> &Users,
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const TargetLibraryInfo *TLI) {
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@ -1887,7 +1895,12 @@ isAllocSiteRemovable(Instruction *AI, SmallVectorImpl<WeakVH> &Users,
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case Instruction::ICmp: {
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ICmpInst *ICI = cast<ICmpInst>(I);
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// We can fold eq/ne comparisons with null to false/true, respectively.
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if (!ICI->isEquality() || !isa<ConstantPointerNull>(ICI->getOperand(1)))
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// We fold comparisons in some conditions provided the alloc has not
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// escaped.
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if (!ICI->isEquality())
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return false;
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unsigned OtherIndex = (ICI->getOperand(0) == PI) ? 1 : 0;
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if (!isNeverEqualToUnescapedAlloc(ICI->getOperand(OtherIndex)))
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return false;
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Users.emplace_back(I);
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continue;
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42
test/Transforms/InstCombine/compare-unescaped.ll
Normal file
42
test/Transforms/InstCombine/compare-unescaped.ll
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@ -0,0 +1,42 @@
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; RUN: opt -instcombine -S < %s | FileCheck %s
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@gp = global i32* null, align 8
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declare i8* @malloc(i64) #1
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define i1 @compare_global_trivialeq() {
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%m = call i8* @malloc(i64 4)
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%bc = bitcast i8* %m to i32*
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%lgp = load i32*, i32** @gp, align 8
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%cmp = icmp eq i32* %bc, %lgp
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ret i1 %cmp
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; CHECK-LABEL: compare_global_trivialeq
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; CHECK: ret i1 false
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}
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define i1 @compare_global_trivialne() {
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%m = call i8* @malloc(i64 4)
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%bc = bitcast i8* %m to i32*
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%lgp = load i32*, i32** @gp, align 8
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%cmp = icmp ne i32* %bc, %lgp
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ret i1 %cmp
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; CHECK-LABEL: compare_global_trivialne
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; CHECK: ret i1 true
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}
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; Although the %m is marked nocapture in the deopt operand in call to function f,
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; we cannot remove the alloc site: call to malloc
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; FIXME: The comparison should fold to false irrespective of whether the call to malloc can be elided or not
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declare void @f()
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define i32 @compare_and_call_with_deopt() {
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; CHECK-LABEL: compare_and_call_with_deopt
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%m = call i8* @malloc(i64 24)
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%bc = bitcast i8* %m to i32*
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%lgp = load i32*, i32** @gp, align 8
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%cmp = icmp eq i32* %bc, %lgp
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%rt = zext i1 %cmp to i32
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tail call void @f() [ "deopt"(i8* %m) ]
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ret i32 %rt
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; CHECK: ret i32 %rt
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}
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