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From: "evan@coeus-group.com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/99698] New: [aarch64] Impossible to accurately detect
 extensions using preprocessor
Date: Sun, 21 Mar 2021 17:01:59 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99698

            Bug ID: 99698
           Summary: [aarch64] Impossible to accurately detect extensions
                    using preprocessor
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: evan@coeus-group.com
  Target Milestone: ---

Using SHA3 as an example, but this problem exists for other extensions as w=
ell.

If you pass something like -march=3Darmv8-a+sha3, GCC will define
__ARM_FEATURE_SHA3.  According to the ACLE documentation:

  __ARM_FEATURE_SHA3 is defined to 1 if the SHA1 & SHA2 Crypto instructions
  from Armv8-A and the SHA2 and SHA3 instructions from Armv8.2-A and newer
  are supported and intrinsics targeting them are available. These
  instructions include AES{E, D}, SHA1{C, P, M}, RAX, and others.

So, I should be able to use the preprocessor to call a function like vbcaxq=
_u32
if __ARM_FEATURE_SHA3 is defined to 1.

However, if I call vbcaxq_u32 I get an error about a target specific option
mismatch.  It turns out that you need -march=3Darmv8.2-a+sha3 to convince G=
CC to
accept the function call.

AFAICT there is no way to reliably detect armv8.2-a.  The only difference in
the preprocessor macros defined for armv8-a and armv8.2-a are a few extra
__ARM_FEATURE_* macros.  There is *no* difference in pre-defined macros bet=
ween
armv8.1-a and armv8.2-a.

FWIW, clang accepts -march=3Darmv8-a+sha3, defines __ARM_FEATURE_SHA3 to 1,=
 and
allows you to call the SHA3 functions, which seems pretty reasonable to me.=