[llvm-bugs] [Bug 49671] New: __ARM_ARCH incorrect for armv8.1-a and above

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49671

            Bug ID: 49671
           Summary: __ARM_ARCH incorrect for armv8.1-a and above
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: clang at evan.coeusgroup.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

According to the ACLE Q3 2020 documentation
(<https://developer.arm.com/documentation/101028/0012>), section 5.4.1
("A32/T32 instruction set architecture"), for 8.1+ __ARM_ARCH is supposed to be
801 for armv8.1-a, 802 for armv8.2-a, 803 for armv8.3-a, etc.

Clang currently just uses 8 for all versions of armv8.  Testing is pretty
straightforward: `clang --target=aarch64-linux-gnu -march=armv8.6-a -dM -E -
</dev/null | grep __ARM_ARCH`.

This makes it difficult to determine support for features which are only
available in later revisions but don't have an __ARM_FEATURE_* macro, such as
complex MLA (8.3).

Similar GCC bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99312

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