Re: [PATCH v3 2/2] x86/lib: Use REP MOVSB for large memcpy() on ERMS CPUs without FSRM

[David Laight <david.laight.linux@gmail.com>]

On Fri, 13 Nov 2026 06:11:36 -0800
Usama Arif <usama.arif@linux.dev> wrote:

> How about something like what I have added at the end, would that be more
> acceptable in terms of maintainability?

I was thinking more of something like:
(written in the email so untested)

static always_inline void *memcpy(void *dst, const void *src, size_t len)
{
	void *orig_dst = dst;

	asm volatile(	".pushsection ".memcpy_calls\n"
			"\t.long . - 1f\n"
			".popsection\n"
			"1:\trep movsb\n"
			"\tnop; nop"
			: "+c (len), "+D" (dst), "+S" (src)
			:: ASM_CALL_CONSTRAINT);

	return orig_dst;
}

An unused return value will be optimised away at all the call sites.

That will work (maybe slowly) on all cpu.
Then some code reads all the addresses from .memcpy_calls and, if necessary,
patches the 5 bytes to a call to the appropriate function.

It might be worth giving the code one or two scratch registers so that
a short copy loop doesn't need to spill to stack (perhaps %rax and %rdx).
It'll matter less for a longer copy loop.

So you might have:
_memcpy_erms:
	cmp $nnnn, %rcx
	jb  _memcpy_words
	rep movsb
	RET

_memcpy_words:
	cmpb $nnnn %rcx
	jb  _memcpy_bytes
	mov %rax,%rcx
	or  %rcx,%rdi
	or  %rcx,%rsi
	test $7,%rcx
	jnz _memcpy_misaligned
	sub $8,%rax
1:	mov %rcx,[%rsi]
	mov %rdx,8[%rsi]
	mov [%rdi],%rcx
	mov 8[%rdi],%rdx
	add $16,%rdi
	add $16,%rsi
	sub $16,%rax
	jnc 1b:
	jz  2f
	mov %rax,[%si]
	mov [%rdi]%rax]
2:	RET

That isn't the best 2-word loop by any means.
And you may not care about misaligned pointers.
	
etc

David