* Linus Torvalds <[email protected]> wrote:
> On Mon, Jun 4, 2018 at 5:21 AM Ingo Molnar <[email protected]> wrote: > > > > - __clear_user() micro-optimization (Alexey Dobriyan) > > Was this actually tested? I'm not sure - Alexey? > 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. Ok, fair point and agreed - if Alexey sends some measurements to back the change I'll keep this, otherwise queue up a revert. Thanks, Ingo
