Messages in this thread | | | Date | Wed, 30 Aug 2023 23:08:36 +0700 | From | Ammar Faizi <> | Subject | Re: [RFC PATCH v1 2/5] tools/nolibc: x86-64: Use `rep stosb` for `memset()` |
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On Wed, Aug 30, 2023 at 05:51:52PM +0200, Willy Tarreau wrote: > Normal, that's because historically "xchg ax, regX" was a single-byte 0x9X > on 8086, then it turned to 32-bit keeping the same encoding, like many > instructions (note that NOP is encoded as xchg ax,ax). It remains short > when you can sacrifice the other register, or restore it later using yet > another xchg. For rcx/rdx a push/pop could do it as they should also be > a single-byte 0x5X even in long mode unless I'm mistaken. Thus if you > absolutely want to squeeze that 9th byte to end up with a 8-byte function > you could probably do: > > xchg %eax, %esi 1 > push %rdx 1 > pop %rcx 1 > push %rdi 1 > rep movsb 2 [sic] > pop %rax 1 > ret 1 > ------------- Total: 8 bytes :-)
Fun!
We're not doing a code golf game, though. So, I think I will leave the "mov %rdx, %rcx" as is. Otherwise, I would be tempted to do that all over the place.
-- Ammar Faizi
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