Messages in this thread | | | Date | Wed, 28 Sep 2022 09:24:04 +0200 | Subject | Re: [PATCH v3] x86, mem: move memmove to out of line assembler | From | Rasmus Villemoes <> |
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On 27/09/2022 23.02, Nick Desaulniers wrote:
> + /* Decide forward/backward copy mode */ > + cmpl dest, src > + jb .Lbackwards_header
I know you're mostly just moving existing code, but for my own education I'd like to understand this.
> + /* > + * movs instruction have many startup latency > + * so we handle small size by general register. > + */ > + cmpl $680, n > + jb .Ltoo_small_forwards
OK, this I get, there's some overhead, and hence we need _some_ cutoff value; 680 is probably chosen by some trial-and-error, but the exact value likely doesn't matter too much.
> + /* > + * movs instruction is only good for aligned case. > + */ > + movl src, tmp0 > + xorl dest, tmp0 > + andl $0xff, tmp0 > + jz .Lforward_movs
But this part I don't understand at all. This checks that the src and dest have the same %256 value, which is a rather odd thing, and very unlikely to ever be hit in practice. I could understand if it checked that they were both 4 or 8 or 16-byte aligned (i.e., (src|dest)&FOO)), or if it checked that they had the same offset within a cacheline [say (src^dest)&0x3f].
Any idea where that comes from? Or am I just incapable of reading x86 asm?
> +.Ltoo_small_forwards: > + subl $0x10, n > + > + /* > + * We gobble 16 bytes forward in each loop. > + */ > +.L16_byteswap_forwards_loop: > + subl $0x10, n > + movl 0*4(src), tmp0 > + movl 1*4(src), tmp1 > + movl tmp0, 0*4(dest) > + movl tmp1, 1*4(dest) > + movl 2*4(src), tmp0 > + movl 3*4(src), tmp1 > + movl tmp0, 2*4(dest) > + movl tmp1, 3*4(dest) > + leal 0x10(src), src > + leal 0x10(dest), dest > + jae .L16_byteswap_forwards_loop > + addl $0x10, n > + jmp .L16_byteswap > + > + /* > + * Handle data forward by movs. > + */ > +.p2align 4 > +.Lforward_movs: > + movl -4(src, n), tmp0 > + leal -4(dest, n), tmp1 > + shrl $2, n > + rep movsl > + movl tmp0, (tmp1) > + jmp .Ldone
So in the original code, %1 was forced to be %esi and %2 was forced to be %edi and they were initialized by src and dest. But here I fail to see how those registers have been properly set up before the rep movs; your names for those are tmp0 and tmp2. You have just loaded the last word of the source to %edi, and AFAICT %esi aka tmp2 is entirely uninitialized at this point (the only use is in L16_byteswap).
I must be missing something. Please enlighten me.
Rasmus
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