Arm CSSC not supported in target_version/target_clones

[tavianator]

Compiling this file

__attribute__((target_clones("cssc", "default")))
int foo(int x) {
    return __builtin_popcount(x);
}

with clang --rtlib=compiler-rt warns that

<source>:1:31: warning: unsupported 'cssc' in the 'target_clones' attribute string; 'target_clones' attribute ignored [-Wignored-attributes]
    1 | __attribute__((target_clones("cssc", "default")))
      |                               ^
<source>:1:31: warning: version list contains entries that don't impact code generation [-Wfunction-multiversion]

However, Clang does understand -march=armv8.7-a+cssc, so it knows about the extension. Is there a different spelling for the target_clones attribute value?

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: Tavian Barnes (tavianator)

Compiling this file

__attribute__((target_clones("cssc", "default")))
int foo(int x) {
    return __builtin_popcount(x);
}

with clang --rtlib=compiler-rt warns that

<source>:1:31: warning: unsupported 'cssc' in the 'target_clones' attribute string; 'target_clones' attribute ignored [-Wignored-attributes]
    1 | __attribute__((target_clones("cssc", "default")))
      |                               ^
<source>:1:31: warning: version list contains entries that don't impact code generation [-Wfunction-multiversion]

However, Clang does understand -march=armv8.7-a+cssc, so it knows about the extension. Is there a different spelling for the target_clones attribute value?

[jroelofs]

target_clones and target_version have a curated list of feature names that they support, listed here: https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/AArch64/AArch64FMV.td

cc @labrinea

[tavianator]

RIght, I did eventually figure that out. (Is it documented somewhere? I think ideally LLVM would document the set of allowable attribute strings.) I sent a feature request to the ACLE here: ARM-software/acle#387

[tavianator]

Fixed by #132727, thanks!