1896fb2cff
This fixes a bug detected in DAGCombiner when using global alias
variables. Here is an example:
@foo = global i16 0, align 1
@aliasFoo = alias i16, i16 * @foo
define i16 @bar() {
...
store i16 7, i16 * @foo, align 1
store i16 8, i16 * @aliasFoo, align 1
...
}
BaseIndexOffset::computeAliasing would incorrectly derive NoAlias
for the two accesses in the example above, resulting in DAGCombiner
miscompiles.
This patch fixes the problem by a defensive approach letting
BaseIndexOffset::computeAliasing return false, i.e. that the aliasing
couldn't be determined, when comparing two global values and at least
one is a GlobalAlias. In the future we might improve this with a
deeper analysis to look at the aliasee for the GlobalAlias etc. But
that is a bit more complicated considering that we could have
'local_unnamed_addr' and situations with several 'alias' variables.
Fixes PR51878.
Differential Revision: https://reviews.llvm.org/D110064
34 lines
1.1 KiB
LLVM
34 lines
1.1 KiB
LLVM
; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
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; RUN: llc -O1 -mtriple i686-unknown-linux-gnu -o - %s | FileCheck %s
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@foo = global i16 0, align 1
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@aliasFoo = alias i16, i16 * @foo
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@bar = global i16 0, align 1
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; This used to miscompile due to not realizing that the store to @aliasFoo
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; clobbered @foo (see PR51878).
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;
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; With some improvements to codegen it should be possible to detect that @foo
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; and @aliasFoo are aliases, and that @aliasFoo isn't aliasing with @bar. So
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; ideally we would end up with three movw instructions here. Running opt
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; before llc on this test case would take care of that, but llc is not smart
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; enough to deduce that itself yet.
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define i16 @main() {
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; CHECK-LABEL: main:
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; CHECK: # %bb.0: # %entry
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; CHECK-NEXT: movw $1, foo
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; CHECK-NEXT: movw $2, bar
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; CHECK-NEXT: movw $4, aliasFoo
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; CHECK-NEXT: movzwl foo, %eax
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; CHECK-NEXT: addw bar, %ax
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; CHECK-NEXT: retl
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entry:
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store i16 1, i16 * @foo
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store i16 2, i16 * @bar
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store i16 4, i16 * @aliasFoo
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%foo = load i16, i16 * @foo
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%bar = load i16, i16 * @bar
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%res = add i16 %foo, %bar
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ret i16 %res
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}
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