On Sun, Jan 21, 2018 at 5:38 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> 3. What's with sbb; and? I can see two sane ways to do this. One is
> cmovaq [something safe], %rax,
Heh. I think it's partly about being old-fashioned. sbb has always
been around, and is the traditional trick for 0/-1.
Also, my original suggested thing did the *access* too, and masked the
result with the same mask.
But I guess we could use cmov instead. It has very similar performance
(ie it was relatively slow on P4, but so was sbb).
However, I suspect it actually has a slightly higher register
pressure, since you'd need to have that zero register (zero being the
"safe" value), and the only good way to get a zero value is the xor
thing, which affects flags and thus needs to be before the cmp.
In contrast, the sbb trick has no early inputs needed.
So on the whole, 'cmov' may be more natural on a conceptual level, but
the sbb trick really is a very "traditional x86 thing" to do.
Linus
