Re: [tip:x86/asm] x86/defconfig: Turn on CONFIG_CC_OPTIMIZE_FOR_SIZE=y in the 64-bit defconfig

[Linus Torvalds]

On Sat, Jan 26, 2013 at 7:18 AM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> On the CPUs Ling is testing on the downsides of -Os probably matter less, in particular since rep movsb works well.
>
> It is questionable as a generic default, though.

So being the person who really pushed for -Os to begin with (I think
I$ and instruction decode bandwidth is one of the most fundamental
limits to CPU performance), I wouldn't mind it if we reintroduced it.

HOWEVER.

It wasn't just "rep movs". The thing that killed -Os for me was that
it makes it impossible to try to optimize hot code, because -Os seems
to throw out branch prediction information. So when you use "likely()"
etc to try to teach the compiler to lay out code a certain way so that
code that never really gets executed isn't even brought into the I$,
-Os then screws it up completely.

Of course, maybe newer versions of gcc might not suck so horribly with
-Os, I haven't actually tried in a while.

[ Just tested. Still does it ]

Also, I doubt Ling was testing a SB CPU. Because "rep movb" still
sucks pretty bad on SB. What core *is* Ling testing? Haswell?

Ugh. We could make it depend on the optimization target. I'd also wish
there was some way to just tune gcc -Os to be closer to reasonable. Or
make -O2 not do some of the excessive crap it does (it aligns code
*much* too much, for example - who cares if you can do it with a
single instruction, if that instruction is so long that it uses up
half your decode bandwidth?)

The problem, of course, is that most -O2 code generation is done
assuming hot loops that don't show much if any I$ issues. And the -Os
thing is done *purely* for size, not taking any performance into
account at all. There's no balanced middle ground, which is what _we_
would want.

                  Linus
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