Yibo Cai created ARROW-15461:
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Summary: [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
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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