[Bug 1884382] Re: __builtin_cpu_supports does not work in ifunc resolver

[Dimitri John Ledkov]

** Attachment added: "test_builtin_ifunc.c"
   https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1884382/+attachment/5385629/+files/test_builtin_ifunc.c

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Title:
  __builtin_cpu_supports does not work in ifunc resolver

Status in gcc-10 package in Ubuntu:
  Invalid
Status in glibc package in Ubuntu:
  Invalid

Bug description:
  I am trying to use ifunc

  On good architectures like armhf & arm64, glibc passes hwcaps argument
  to the ifunc resolver function, which one can use to test for various
  features.

  On x86_64, no argument is passed, thus I'm trying to use
  __builtin_cpu_supports() function instead. It would be nice if glibc
  did pass something like whatever __builtin_cpu_supports() needs.

  However, it seems like __builtin_cpu_supports is unusable from inside
  ifunc resolver. Re-implementing __builtin_cpu_supports with
  __get_cpuid works.

  Using __builtin_cpu_supports in main() otherwise works.

  Is my code wrong, or can gcc/glibc be improved, such that
  __builtin_cpu_supports works in an ifunc resolver?

  See attached code.

  $ gcc-10 test_builtin_ifunc.c -o test_builtin_ifunc
  $ ./test_builtin_ifunc
  sse41
  portable
  sse41

  Whereas on a machine that supports SSE4.1 the expected output is
  sse41
  sse41
  sse41

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