Messages in this thread | | | Date | Fri, 6 Jan 2006 01:54:44 +0100 | From | Ingo Molnar <> | Subject | Re: [patch 00/2] improve .text size on gcc 4.0 and newer compilers |
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* Linus Torvalds <torvalds@osdl.org> wrote:
> > Especially if there enough profiling hits, it's usually a quick glance > > to figure out the hotpath: > > Ehh. What's a "quick glance" to a human can be quite hard to automate. > That's my point. > > If we do the "human quick glances", we won't be seeing much come out > of this. That's what we've already been doing, for several years. > > I thought the discussion was about trying to automate this..
i think it could be automated reasonably well. An 80% effective "which condition is judged incorrectly" decision could be made based on:
branch instruction with more than 10% of the average per-instruction cycle count, followed by an at least 4-instruction sequence of non-branch (and non-jump) instructions that have exactly zero profiling hits. ('few hits' we are not interested in - those are not likely/unlikely candidates)
another part is to feed this back into .c, automatically. I've done DEBUG_INFO, gdb vmlinux and list *0x12341234 based scripts before, but they are not always reliable. They could probably do something like: "if the resulting source code contains a clear 'if (' sequence, modify it to 'if __unlikely (', or something like that.
i'd expect such a method to catch ~50-60% of the interesting cases, not more. (the rest would be stuff the heuristics doesnt catch, and it would also be stuff like 'while' or 'break' or 'goto', which are much harder to rewrite automatically.
but it does feel quite a bit fragile.
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