[ARROW-15461] [C++] arrow-utility-test fails with clang-12 (TestCopyAndReverseBitmapPreAllocated)

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 7.0.0
• Component/s: C++
• Labels: pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/30939
• Assignee: Antoine Pitrou
• Reporter: Yibo Cai
• Created: 26/Jan/22 11:34
• Updated: 11/Jan/23 11:37
• Resolved: 28/Jan/22 04:42

Description

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

Issue Links

• links to: GitHub Pull Request #12276