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3b948587b2
no-alias with non-addr-taken globals: they cannot alias a captured pointer. If the non-global underlying object would have been a capture were it to alias the global, we can firmly conclude no-alias. It isn't reasonable for a transformation to introduce a capture in a way observable by an alias analysis. Consider, even if it were to temporarily capture one globals address into another global and then restore the other global afterward, there would be no way for the load in the alias query to observe that capture event correctly. If it observes it then the temporary capturing would have changed the meaning of the program, making it an invalid transformation. Even instrumentation passes or a pass which is synthesizing stores to global variables to expose race conditions in programs could not trigger this unless it queried the alias analysis infrastructure mid-transform, in which case it seems reasonable to return results from before the transform started. See the comments in the change for a more detailed outlining of the theory here. This should address the primary performance regression found when the non-conservatively-correct path of the alias query was disabled. Differential Revision: http://reviews.llvm.org/D11410 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243405 91177308-0d34-0410-b5e6-96231b3b80d8
63 lines
1.6 KiB
LLVM
63 lines
1.6 KiB
LLVM
; RUN: opt < %s -globalsmodref-aa -gvn -S | FileCheck %s
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;
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; This tests the safe no-alias conclusions of GMR -- when there is
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; a non-escaping global as one indentified underlying object and some pointer
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; that would inherently have escaped any other function as the other underlying
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; pointer of an alias query.
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@g1 = internal global i32 0
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define i32 @test1(i32* %param) {
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; Ensure that we can fold a store to a load of a global across a store to
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; a parameter when the global is non-escaping.
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;
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; CHECK-LABEL: @test1(
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; CHECK: store i32 42, i32* @g1
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; CHECK-NOT: load i32
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; CHECK: ret i32 42
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entry:
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store i32 42, i32* @g1
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store i32 7, i32* %param
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%v = load i32, i32* @g1
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ret i32 %v
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}
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declare i32* @f()
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define i32 @test2() {
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; Ensure that we can fold a store to a load of a global across a store to
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; the pointer returned by a function call. Since the global could not escape,
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; this function cannot be returning its address.
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;
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; CHECK-LABEL: @test2(
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; CHECK: store i32 42, i32* @g1
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; CHECK-NOT: load i32
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; CHECK: ret i32 42
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entry:
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%ptr = call i32* @f() readnone
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store i32 42, i32* @g1
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store i32 7, i32* %ptr
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%v = load i32, i32* @g1
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ret i32 %v
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}
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@g2 = external global i32*
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define i32 @test3() {
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; Ensure that we can fold a store to a load of a global across a store to
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; the pointer loaded from that global. Because the global does not escape, it
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; cannot alias a pointer loaded out of a global.
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;
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; CHECK-LABEL: @test3(
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; CHECK: store i32 42, i32* @g1
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; CHECK: store i32 7, i32*
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; CHECK-NOT: load i32
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; CHECK: ret i32 42
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entry:
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store i32 42, i32* @g1
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%ptr1 = load i32*, i32** @g2
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store i32 7, i32* %ptr1
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%v = load i32, i32* @g1
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ret i32 %v
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}
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