Hi Brian,
I just wanted to add that the stash and store idea you suggest that is also
used in memmove has a very nice impact
on the assembly code.
With the old code that does this for the last 0 to 7 words:
while (candidate_ptr > score_ptr) {
*candidate_ptr = *(candidate_ptr - 1);
candidate_ptr--;
}
the assembly code shows this from the point where the move starts:
.L24:
movdqu -16(%rax), %xmm1
subq $16, %rax
movups %xmm1, 2(%rax)
cmpq %rdx, %rax
jnb .L24
movq %r10, %rax
subq %r9, %rax
subq $16, %rax
notq %rax
andq $-16, %rax
addq %r10, %rax
cmpq %rax, %r9
jnb .L28
movq %rax, %rcx
movq %rax, %rdx
movq %r9, 48(%rsp)
subq %r9, %rcx
subq $1, %rcx
shrq %rcx
leaq 2(%rcx,%rcx), %r8
negq %rcx
subq %r8, %rdx
leaq (%rax,%rcx,2), %rcx
call memmove
movq 48(%rsp), %r9
jmp .L28
But with stash and store:
*(uint64_t *)&candidates_index[new_score_rank + 1] = first_four;
*(uint64_t *)&candidates_index[new_score_rank + 5] = next_four;
the assembly code from the point where the move start is this:
.L24:
movdqu -16(%r9), %xmm1
subq $16, %r9
movups %xmm1, 2(%r9)
cmpq %rax, %r9
jnb .L24
movups %xmm0, 2(%rdi,%rdx)
jmp .L26
There are a couple of extra assembly instructions to stash into xmm0 before the
move, but this is a big reduction in
assembly code size for the backward memory move. Not as fast as memmove if the
DF wasn't getting corrupted, but much
better than the old code plus it completely avoids the risk of DF corruption
during rep movsq in memmove for backward
move sizes >= 8! I like it because there is no need to worry about whether rep
movsb or rep movsw could also be
vulnerable to DF corruption.
Best Regards,
Kennon
> On 02/27/2026 11:49 AM PST Brian Inglis via Cygwin <[email protected]> wrote:
>
>
> Hi Kennon,
>
> Some perf reports and analysis imply that backward moves (with overlap?) are
> no
> faster than straight rep movsb on some CPUs, so it may be better to just
> simplify to that, unless you want to stash the final element(s) to be moved
> out
> of the way in register(s), and use multiple registers in unrolled wide moves
> for
> the aligned portion?
>
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