[PATCH] D105462: [X86] Add CRC32 feature.

[Wang Tianqing via Phabricator via cfe-commits]

tianqing added inline comments.


================
Comment at: clang/lib/Basic/Targets/X86.cpp:159
+  // Enable CRC32 if SSE4.2 is enabled.
+  // NOTE: In conformance with GCC behavior, CRC32 is still available even if
+  // it's explicitly disabled.
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craig.topper wrote:
> This doesn't seem to be true. It causes gcc to crash. https://godbolt.org/z/39rEbsejh
Well I was using GCC 11.1, it compiles.

The way I see it, crash means a bug (not surprising since it's trunk), and can be interpreted as incompletely defined behavior until it's fixed.

Some tests on GCC trunk:
1. -msse4.2: Pass - sse4.2 enables crc32.
2. -mcrc32 -mno-sse4.2: Pass - no-sse4.2 doesn't disable crc32.
3. -msse4.2 -mno-sse4.2: Error - no-sse4.2 disables crc32.
4. -mno-crc32 -msse4.2: Crash - undefined behavior
5. -msse4.2 -mno-crc32: Crash - undefined behavior


It's hard to extract some consistent underlying logic from the GCC results.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D105462/new/

https://reviews.llvm.org/D105462