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If a local_unnamed_addr attribute is attached to a global, the address is known to be insignificant within the module. It is distinct from the existing unnamed_addr attribute in that it only describes a local property of the module rather than a global property of the symbol. This attribute is intended to be used by the code generator and LTO to allow the linker to decide whether the global needs to be in the symbol table. It is possible to exclude a global from the symbol table if three things are true: - This attribute is present on every instance of the global (which means that the normal rule that the global must have a unique address can be broken without being observable by the program by performing comparisons against the global's address) - The global has linkonce_odr linkage (which means that each linkage unit must have its own copy of the global if it requires one, and the copy in each linkage unit must be the same) - It is a constant or a function (which means that the program cannot observe that the unique-address rule has been broken by writing to the global) Although this attribute could in principle be computed from the module contents, LTO clients (i.e. linkers) will normally need to be able to compute this property as part of symbol resolution, and it would be inefficient to materialize every module just to compute it. See: http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160509/356401.html http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160516/356738.html for earlier discussion. Part of the fix for PR27553. Differential Revision: http://reviews.llvm.org/D20348 llvm-svn: 272709
80 lines
2.1 KiB
LLVM
80 lines
2.1 KiB
LLVM
; RUN: opt -S -globalopt < %s | FileCheck %s
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; This test is hint, what could globalOpt optimize and what it can't
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; FIXME: @tmp and @tmp2 can be safely set to 42
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; CHECK: @tmp = local_unnamed_addr global i32 0
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; CHECK: @tmp2 = local_unnamed_addr global i32 0
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; CHECK: @tmp3 = global i32 0
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@tmp = global i32 0
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@tmp2 = global i32 0
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@tmp3 = global i32 0
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@ptrToTmp3 = global i32* null
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@llvm.global_ctors = appending global [1 x { i32, void ()* }] [{ i32, void ()* } { i32 65535, void ()* @_GLOBAL__I_a }]
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define i32 @TheAnswerToLifeTheUniverseAndEverything() {
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ret i32 42
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}
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define void @_GLOBAL__I_a() {
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enter:
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call void @_optimizable()
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call void @_not_optimizable()
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ret void
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}
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define void @_optimizable() {
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enter:
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%valptr = alloca i32
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%val = call i32 @TheAnswerToLifeTheUniverseAndEverything()
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store i32 %val, i32* @tmp
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store i32 %val, i32* %valptr
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%0 = bitcast i32* %valptr to i8*
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%barr = call i8* @llvm.invariant.group.barrier(i8* %0)
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%1 = bitcast i8* %barr to i32*
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%val2 = load i32, i32* %1
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store i32 %val2, i32* @tmp2
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ret void
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}
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; We can't step through invariant.group.barrier here, because that would change
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; this load in @usage_of_globals()
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; val = load i32, i32* %ptrVal, !invariant.group !0
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; into
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; %val = load i32, i32* @tmp3, !invariant.group !0
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; and then we could assume that %val and %val2 to be the same, which coud be
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; false, because @changeTmp3ValAndCallBarrierInside() may change the value
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; of @tmp3.
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define void @_not_optimizable() {
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enter:
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store i32 13, i32* @tmp3, !invariant.group !0
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%0 = bitcast i32* @tmp3 to i8*
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%barr = call i8* @llvm.invariant.group.barrier(i8* %0)
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%1 = bitcast i8* %barr to i32*
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store i32* %1, i32** @ptrToTmp3
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store i32 42, i32* %1, !invariant.group !0
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ret void
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}
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define void @usage_of_globals() {
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entry:
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%ptrVal = load i32*, i32** @ptrToTmp3
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%val = load i32, i32* %ptrVal, !invariant.group !0
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call void @changeTmp3ValAndCallBarrierInside()
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%val2 = load i32, i32* @tmp3, !invariant.group !0
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ret void;
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}
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declare void @changeTmp3ValAndCallBarrierInside()
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declare i8* @llvm.invariant.group.barrier(i8*)
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!0 = !{!"something"}
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