Messages in this thread | | | From | Linus Torvalds <> | Date | Mon, 4 Jun 2018 18:58:56 -0700 | Subject | Re: [GIT PULL] x86/asm changes for v4.18 |
| |
On Mon, Jun 4, 2018 at 5:21 AM Ingo Molnar <mingo@kernel.org> wrote: > > - __clear_user() micro-optimization (Alexey Dobriyan)
Was this actually tested?
I think one reason people avoided the constant was that on some microarchitecture it ended up being a separate uop just for the constant generation, because it wouldn't fit in a single uop.
I'm pretty sure that used to be the case for P4, for example.
Afaik there have also been issues with decoding instructions that have both an immediate and a memory offset.
I suspect none of this is an issue on modern cores, but there really at least historically were cases where
mov %reg,mem
was better than
mov $imm,mem
if %reg already had the right value, so it's not at all 100% obvious that the micro-optimization really _optimizes_ anything.
Any time people do this, they should add numbers.
Linus
| |