[llvm-bugs] [Bug 42982] New: r368276 caused msan false positive in vectorized crc code

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42982

            Bug ID: 42982
           Summary: r368276 caused msan false positive in vectorized crc
                    code
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: hans at chromium.org
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk

Created attachment 22372
  --> https://bugs.llvm.org/attachment.cgi?id=22372&action=edit
reproducer

Building the attached source code at r368276:

$ /work/llvm.monorepo/build.release/bin/clang.bad crc32_simd.c -msse4.2
-mpclmul -g -O2 -fsanitize=memory
$ MSAN_SYMBOLIZER_PATH=/work/llvm.monorepo/build.release/bin/llvm-symbolizer
./a.out
==87075==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x4a2c06 in crc32_sse42_simd_ /tmp/crc32_simd.c:141:10
    #1 0x4a2c06 in main /tmp/crc32_simd.c:163:10
    #2 0x7fed1ff0152a in __libc_start_main
(/lib/x86_64-linux-gnu/libc.so.6+0x2352a)
    #3 0x41f299 in _start (/tmp/a.out+0x41f299)

SUMMARY: MemorySanitizer: use-of-uninitialized-value /tmp/crc32_simd.c:141:10
in crc32_sse42_simd_
Exiting


The warning didn't fire before, and I don't see anything wrong with the code.
This suggests perhaps the transformation in r368660 is not correct, or exposed
some previous problem. It was reverted in r368660.



(This was discovered in Chromium, see https://crbug.com/992853)

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