Re: [x86/asm] 0507503671: will-it-scale.per_process_ops -4.9% regression

[Peter Zijlstra <peterz@infradead.org>]

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On Mon, Nov 15, 2021 at 01:15:49PM -0800, H. Peter Anvin wrote:
> It is perhaps a bit hard for gcc to know what S's are C so they can be E'd, since all it sees is assembly.

Well, if we were able to declare it pure, then it would know that the
output only depends on the inputs and thus it can merge all
static_call_{,un}likely() branches that take the same key argument. But
alas.

Funnily, everything I try to make that function 'better' actually makes
it longer, so I'm suspecting it does something clever with
static_cpu_has() nevertheless.

> It also doesn't explain how this code
> could possibly have this kind of impact; of anything, it should make this change more beneficial, not less; certainly not make it consume 5% more CPU.

Yeah, no idea there. We've had wild 0day reports before due to either
code or data layout changes that otherwise make no sense at all. They've
tried to eliminate a bunch of that by increasing function alignment or
somesuch (there was a talk at LPC? on that). But it remains a bit of a
mystery.