Messages in this thread | | | Date | Sat, 11 Apr 2015 12:35:58 -0700 | Subject | Re: [PATCH] x86: Turn off GCC branch probability heuristics | From | Linus Torvalds <> |
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On Sat, Apr 11, 2015 at 11:57 AM, Thomas Gleixner <tglx@linutronix.de> wrote: > > I thinks its just the no-guess one: > > text data dec patch reduction > 7563475 1781048 10302987 > 7192973 1780024 9931461 no-guess -4.8% > 7354819 1781048 958464 align-1 -2.7% > 7192973 1780024 9931461 no-guess + align-1 -4.8%
Yeah, a 5% code expansion is a big deal. Sadly, it looks like 'no-guess' also disables our explicit likely/unlikely handling.
Damn. If it actually honored likely/unlikely, then we should just do it - and manually fix up any places where we really care.
But the fact that it apparently entirely disables not just the guesses, but our *explicit* likely/unlikely, means that we can't fix up the mistakes.
And in many of the hot codepaths that likely/unlikely really does matter. Some of our hottest paths have known "this basically never happens" situations that we do *not* want to break up our L1 I$ over. There's a number of functions that have been optimized to really generate good code, and "-fno-guess-branch-probability" disables those manual optimizations.
So we'd have no way to fix it for the cases that matter.
Sad.
It might be worth bringing this up with some gcc people. I added Jakub to the cc. Any other gcc people suggestions?
Linus
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