[llvm] [InstSimplify] Correctly handle comparison with zero-size allocs (PR #115728)

[Nikita Popov via llvm-commits]

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@@ -249,8 +250,10 @@ define i1 @zst_alloca_middle() {
 ; CHECK-LABEL: @zst_alloca_middle(
 ; CHECK-NEXT:    [[A:%.*]] = alloca i64, align 8
 ; CHECK-NEXT:    [[A2:%.*]] = alloca {}, align 8
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i8, ptr [[A]], i64 4
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq ptr [[GEP]], [[A2]]
 ; CHECK-NEXT:    call void @escape(ptr [[A]], ptr [[A2]])
-; CHECK-NEXT:    ret i1 false
+; CHECK-NEXT:    ret i1 [[CMP]]
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nikic wrote:

Oops, this was actually a mistake in the test. The zero-size allocation was supposed to have `align 1`. It's weird that LLVM infers `align 8` for it by default. I adjusted the test.

InstCombine actually fails to handle this (see https://llvm.godbolt.org/z/WMPKdnrMd for variant without allocas) due to a weakness in foldICmpUsingKnownBits. I'll work on a fix.


https://github.com/llvm/llvm-project/pull/115728