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load-store-vectorizer cannot assume that an offset calculated from add nuw is fine in general #45936

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aqjune opened this issue Jul 5, 2020 · 2 comments
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load-store-vectorizer cannot assume that an offset calculated from add nuw is fine in general #45936

aqjune opened this issue Jul 5, 2020 · 2 comments
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bugzilla Issues migrated from bugzilla loopoptim miscompilation

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@aqjune
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aqjune commented Jul 5, 2020

Bugzilla Link 46591
Version trunk
OS All
Attachments Input & output
CC @efriedma-quic,@fhahn,@arsenm,@nunoplopes

Extended Description

test/Transforms/LoadStoreVectorizer/X86/vectorize-i8-nested-add.ll 's ld_v4i8_add_nuw vectorizes a sequence of loads, which is illegal.

https://godbolt.org/z/NzK5oY
(the attached file contains this input/output)

Counter example: if v0 and v1 is 0, nuw is never triggered, but the offsets aren't still consecutive.
https://alive2.llvm.org/ce/z/4nZKBs

This can happen when a program has an allocation larger than 4G.
@efriedma-quic
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I briefly looked at Vectorizer::lookThroughComplexAddresses . I think what's happening is that it's using getSExtValue() incorrectly: essentially, it's accidentally treating "zext(x+c)" as if it were "zext(x)+sext(c)", as opposed to "zext(x)+zext(c)". That throws off the rest of the math.

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 10, 2021
@nunoplopes nunoplopes mentioned this issue May 26, 2022
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@nunoplopes
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I think this is fixed (by 2be0abb)
The transformation is no longer performed.

cc @jlebar

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@nunoplopes @aqjune @efriedma-quic