[llvm-bugs] [Bug 46674] New: support for `.arch_extension rcpc`

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=46674

            Bug ID: 46674
           Summary: support for `.arch_extension rcpc`
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: AArch64
          Assignee: unassignedbugs at nondot.org
          Reporter: ndesaulniers at google.com
                CC: arnaud.degrandmaison at arm.com, caij2003 at gmail.com,
                    daniel.kiss at arm.com, llozano at chromium.org,
                    llvm-bugs at lists.llvm.org, oliver.stannard at arm.com,
                    smithp352 at googlemail.com, srhines at google.com,
                    t.p.northover at gmail.com, Ties.Stuij at arm.com
            Blocks: 4068

Proposed kernel patch [0] looks to make use of `rcpc` ARMv8.3 extension [1]. 
It uses feature detection at build time to see if the assembler supports the
following assembler directive:

$ clang -x assembler --target=aarch64-linux-gnu -march=armv8.3a <(echo
".arch_extension rcpc")

I've tried:
* -march=armv8.3a
* -mcpu=cortex-a77+rcpc
* -mcpu=neoverse-e1
* -mcpu=neoverse-n1

But all of them report:

/proc/self/fd/11:1:17: error: unknown architectural extension: rcpc
.arch_extension rcpc
                ^

Looking into llvm unit tests, I don't see any tests that actually test the
assembler for `.arch_extension rcpc`.

[0] https://lore.kernel.org/lkml/20200710165203.31284-18-will@kernel.org/T/#u
[1]
https://sourceware.org/binutils/docs/as/AArch64-Extensions.html#AArch64-Extensions


Referenced Bugs:

https://bugs.llvm.org/show_bug.cgi?id=4068
[Bug 4068] [Meta] Compiling the Linux kernel with clang
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