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

["H. Peter Anvin" <hpa@zytor.com>]

The fast rep movsb was introduced on Ivy Bridge, IIRC.

Linus Torvalds <torvalds@linux-foundation.org> wrote:

>On Sat, Jan 26, 2013 at 7:18 AM, H. Peter Anvin <hpa@zytor.com> 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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