LLVM can't support partial register clobbers

On x86_64-pc-windows-msvc, the system ABI considers xmm0-5 as volatile/caller-save registers, and xmm6-15 as non-volatile/callee-save registers. However, clobber_abi("C") incorrectly marks xmm6-15 as clobbered, which produces a bunch of wasteful register saves/restores.

This behaves according to the documentation (which specifies that xmm6-15 will be clobbered), but it's almost certainly not what the developer wanted due to the unnecessary clobbering causing performance problems. The workaround is to avoid using clobber_abi on Windows and to manually clobber registers; this is a pain and may cause problems down the line as more volatile registers are added to the ABI.

So a breaking change seems warranted here.

Godbolt link

#[no_mangle]
pub fn foo() {
    unsafe {
        std::arch::asm! {
            "call bar",
            clobber_abi("C"),
        }
    }
}

Asm:

foo:
        sub     rsp, 168
        movaps  xmmword ptr [rsp + 144], xmm15
        movaps  xmmword ptr [rsp + 128], xmm14
        movaps  xmmword ptr [rsp + 112], xmm13
        movaps  xmmword ptr [rsp + 96], xmm12
        movaps  xmmword ptr [rsp + 80], xmm11
        movaps  xmmword ptr [rsp + 64], xmm10
        movaps  xmmword ptr [rsp + 48], xmm9
        movaps  xmmword ptr [rsp + 32], xmm8
        movaps  xmmword ptr [rsp + 16], xmm7
        movaps  xmmword ptr [rsp], xmm6

        call    bar

        movaps  xmm6, xmmword ptr [rsp]
        movaps  xmm7, xmmword ptr [rsp + 16]
        movaps  xmm8, xmmword ptr [rsp + 32]
        movaps  xmm9, xmmword ptr [rsp + 48]
        movaps  xmm10, xmmword ptr [rsp + 64]
        movaps  xmm11, xmmword ptr [rsp + 80]
        movaps  xmm12, xmmword ptr [rsp + 96]
        movaps  xmm13, xmmword ptr [rsp + 112]
        movaps  xmm14, xmmword ptr [rsp + 128]
        movaps  xmm15, xmmword ptr [rsp + 144]
        add     rsp, 168
        ret

I expected that foo would just establish a frame and call bar without saving/restoring of the XMM registers.

Meta

rustc --version --verbose:

rustc 1.78.0 (9b00956e5 2024-04-29)
binary: rustc
commit-hash: 9b00956e56009bab2aa15d7bff10916599e3d6d6
commit-date: 2024-04-29
host: x86_64-unknown-linux-gnu
release: 1.78.0
LLVM version: 18.1.2

But this repros in nightly.

Start with the inline assembly clobber_abi("C") handling for the x86_64-pc-windows-msvc target and compare it with the documented Windows ABI. Reproduce the Godbolt example and inspect the generated assembly, then verify that the resulting clobber set no longer causes unnecessary saves and restores of xmm6-15.