Is there any way to force Go to call asm function using register-based arguments?

[Richiise Nugraha]

Hi i wonder if there's option to use register-based arguments, maybe like compiler directive or something?

Expected result:

But this one wasting r9 as function address.

What i got so far:

But this is stack-based :/

[Ian Lance Taylor]

Please always show text as plain text, not as images.  Plain text is much easier to read.  I'm not even going to try to read your images; sorry.

That said, there is no supported way to do this.  You can indicate that an assembly function uses the internal ABI, which is currently the register ABI, by changing "(SB)" to "<ABInternal>" in the TEXT directive introducing the function.  However, as you can guess from the name, the internal ABI is subject to change over time.  See https://go.googlesource.com/proposal/+/refs/heads/master/design/27539-internal-abi.md.  So while it is possible to write such an assembly function today, it may break in future releases.

Ian

[drc...@google.com]

You could use build tags to write a portable implementation and a (very) version-specific implementation.

E.g.

//go:build go1.18 && !go1.19

// go1.18-specific

and

//go:build (go1.8 && !go1.18) || go1.19

// not go1.18

We have ideas about changing the ABI in the future -- possible changes include

- use the closure context pointer to pass the receiver to methods

- don't reserve stack space for spilling in the prologue

- perhaps reserve some registers for generational GC, if we ever do that

[Jakob Thomsen]

Hi all!

Thanks for the pointer to using <ABIInternal> in the code. Unfortunately, it seems that it is only allowed for runtime* packages, as I get a 

ABI selector only permitted when compiling runtime, reference was to "\"\".__CompareNMask"
error, when using it. I'm trying to use it for this simple function (invoking some SIMD operations):

// func __CompareNMask(buf, mask unsafe.Pointer) (ret uint16)
TEXT ·__CompareNMask<ABIInternal>(SB), $0-24

    MOVQ buf+0(FP), DI
    MOVQ mask+8(FP), SI

    LONG $0xc66ef9c5             // vmovd    xmm0, esi
    LONG $0x7879e2c4; BYTE $0xc0 // vpbroadcastb    xmm0, xmm0
    LONG $0x0774f9c5             // vpcmpeqb    xmm0, xmm0, oword [rdi]
    LONG $0xc0d7f9c5             // vpmovmskb    eax, xmm0
    MOVW AX, ret+16(FP)
    RET

Is there any way to allow non-runtime packages to use the register-based calling convention (or open up for the ABIInternal)?

Best regards

Jakob

[opennota]

Any update on this? I wanted to use the register-based calling convention as well and got "ABI selector only permitted when compiling runtime", too.

[opennota]

Ok Google, so one can use `-asmflags=-compiling-runtime` to work around this limitation. The resulting speed-up can be well worth it.

[Keith Randall]

There is still no way to do this in a supported manner.

We may at some future time freeze ABIInternal and call it ABI1, at which point you could use that. But we have no plans or schedule for that at the moment.