[PATCH 4/5] arm64: lib: Use MOPS for memcpy() routines

[Catalin Marinas]

On Mon, Sep 30, 2024 at 05:10:50PM +0100, Kristina Martsenko wrote:
> diff --git a/arch/arm64/lib/memcpy.S b/arch/arm64/lib/memcpy.S
> index 4ab48d49c451..9b99106fb95f 100644
> --- a/arch/arm64/lib/memcpy.S
> +++ b/arch/arm64/lib/memcpy.S
> @@ -57,7 +57,7 @@
>     The loop tail is handled by always copying 64 bytes from the end.
>  */
>  
> -SYM_FUNC_START(__pi_memcpy)
> +SYM_FUNC_START_LOCAL(__pi_memcpy_generic)
>  	add	srcend, src, count
>  	add	dstend, dstin, count
>  	cmp	count, 128
> @@ -238,7 +238,24 @@ L(copy64_from_start):
>  	stp	B_l, B_h, [dstin, 16]
>  	stp	C_l, C_h, [dstin]
>  	ret
> +SYM_FUNC_END(__pi_memcpy_generic)
> +
> +#ifdef CONFIG_AS_HAS_MOPS
> +	.arch_extension mops
> +SYM_FUNC_START(__pi_memcpy)
> +alternative_if_not ARM64_HAS_MOPS
> +	b	__pi_memcpy_generic
> +alternative_else_nop_endif

I'm fine with patching the branch but I wonder whether, for the time
being, we should use alternative_if instead and the NOP to fall through
the default implementation. The hardware in the field doesn't have
FEAT_MOPS yet and they may see a slight penalty introduced by the
branch, especially for small memcpys. Just guessing, I haven't done any
benchmarks.

-- 
Catalin