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https://issues.apache.org/jira/browse/ARROW-15461?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17483111#comment-17483111
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Yibo Cai commented on ARROW-15461:
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Great! Compiler generates much better machine code.
Recommend opening a PR for this change.
Curious why current c code is not optimized this way, given (len > 8) is UB.
> [C++] arrow-utility-test fails with clang-12
> (TestCopyAndReverseBitmapPreAllocated)
> -----------------------------------------------------------------------------------
>
> Key: ARROW-15461
> URL: https://issues.apache.org/jira/browse/ARROW-15461
> Project: Apache Arrow
> Issue Type: Bug
> Components: C++
> Reporter: Yibo Cai
> Priority: Major
>
> Unit test {{BitUtilTests.TestCopyAndReverseBitmapPreAllocated}} failed if
> release build arrow with clang-12, on both x86 and Arm.
> Per my debug, it's related to {{GetReversedBlock}} function [1], when right
> shift a uint8 value by 8 bits.
> I think it's a compiler bug. From the test code [2], clang-12 returns 1,
> which is wrong. clang-11 and clang-13 both return 2, the correct answer.
> Looks clang-12 over optimized the code, there should be no UB in the code
> (uint8 is promoted to integer before shift).
> A workaround is to treat shifting 8 bits as a special case. Or we can simply
> ignore this error if the compiler bug is confirmed (I didn't find clang bug
> report).
> [1]
> https://github.com/apache/arrow/blob/master/cpp/src/arrow/util/bitmap_ops.cc#L101
> [2] https://godbolt.org/z/TzYWfcP1E
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