On 7/6/2022 7:26 AM, Marek Polacek wrote:
On Tue, Jul 05, 2022 at 03:47:26PM -0400, Matthias Clasen wrote:
> On Tue, Jul 5, 2022 at 3:40 PM Marek Polacek <polacek(a)redhat.com> wrote:
>
>> Maybe not, but even ~1% is still an unacceptable slowdown. It would take
>> about a year for the compiler to catch up.
>>
>>
> (Un)acceptable for whom?
GCC maintainers in Fedora, at least.
> And why would it be unacceptable?
Because it's too much.
> You just said compilers will make up for it quickly, not to mention
> hardware continuously getting faster too...
Dozens of developers working a whole release (if not more) is not quick.
> I haven't seen any convincing arguments as to why such a small
> drop would be the end of the world.
And likewise, I haven't seen how this proposal would be helpful to the
majority of users, nevermind that it'd likely break programs using
inline assembly that use %rbp. But others have already raised similar
points in this thread.
> And I don't think Fedora is or should be used in high-speed trading or
> similar
> environments where every microsecond matters.
I think you may be underestimating how much even 1% matters.
Amen. A 1% hit is
very significant -- as Marek indicated, that's
roughly a year of work for the GCC community to recover.
Sometimes we're willing to take a 1% hit, sometimes not. It's a
question of balancing the performance hit against the benefits of
whatever change is being considered.
If I'm understanding things correctly, the original proposal is trying
to make a very special case of profiling work better -- a case that
99.9% of Fedora users do not need or care about. That seems like a
particularly bad cost/benefit for this proposal.
Jeff