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SubjectRe: [PATCH] x86: Align jump targets to 1 byte boundaries

* H. Peter Anvin <hpa@zytor.com> wrote:

> On 04/10/2015 05:50 AM, Denys Vlasenko wrote:
> >
> > However, I'm an -Os guy. Expect -O2 people to disagree :)
> >
>
> The problem with -Os is that the compiler will make *any* tradeoffs
> to save a byte. It is really designed to squeeze as much code into
> a fixed-size chunk, e.g. a ROM, as possible.
>
> We have asked for an -Okernel mode from the gcc folks forever. It
> basically would mean "-Os except when really dumb."

Yes, and it appears that not aligning to 16 bytes gives 5.5% size
savings already - which is a big chunk of the -Os win.

So we might be able to get a "poor man's -Okernel" by not aligning.
(I'm also looking at GCC options to make loop unrolls less aggressive
- that's another common source of bloat.)

I strongly suspect it's the silly 'use weird, wildly data-dependent
instructions just to save a single byte' games are that are killing
-Os performance in practice.

> As far as the 16-byte alignment, my understanding is not that it is
> related to the I$ but rather is the decoder datum.

Yeah, but the decoder stops if the prefetch crosses a cache line? So
it appears to be an interaction of the 16 byte prefetch window and
cache line boundaries?

Btw., given that much of a real life kernel's instructions execute
cache-cold, a 5.5% reduction in kernel size could easily speed up
cache-cold execution by a couple of percent. In the cache-cold case
the prefetch window size is probably not important at all, what
determines execution speed is cache miss latency and cache footprint.

[ At least in my simple mental picture of it, which might be wrong ;-) ]

Thanks,

Ingo


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