Messages in this thread | | | Date | Thu, 10 Jun 2010 18:24:53 +0200 | From | Andi Kleen <> | Subject | Re: [PATCH 03/13] jump label v9: x86 support |
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On Thu, Jun 10, 2010 at 05:37:42PM +0200, Ingo Molnar wrote: > > > [...] It costs you in some benchmarks. [...] > > Microbenchmarks mostly, see below.
I didn't make these decisions, but I assume who made them had good reasons and enough data on larger benchmarks too.
> > A much better to get smaller kernel images is to do more __cold annotations > > for slow paths. Newer gcc will then simply only do -Os for these functions. > > That's an opt-in method and we cannot reach the kinds of 30% code size > reductions that -Os can achieve. Most code in the kernel is not cache-hot, > even on microbenchmarks.
Maybe, maybe not. But yes it can be approached from both ways.
Personally I would prefer to simply write less bloated code to get code reductions. Simpler code is often faster too.
> > A much better model would be to actively mark hot codepaths with a __hot > attribute instead. Then the code size difference can be considered on a case > by case basis.
Yes that works too for those who still use -Os.
e.g. marking the scheduler and a few mm hot paths this way would certain make sense.
> > And where GCC produces indefensibly crap code there GCC needs to be fixed. > Crap code often increases size so the fix would increase the efficiency of > -Os.
In some cases agreed, but common cases it's really: you asked for the smallest you got it, even if it's slow. It's not -Odwim.
One standard example here is a division by constant. The shortest way is using DIVI/IDIV if it's not 2^n and small enough, but it's really quite slow in hardware. If you spend a few more bytes you can do much better for a wide range of constants.
Most likely we would need a new -O flag to avoid such cases.
BTW I experimented with marking a few common cases like this (e.g. time unit conversion) hot, but gcc currently has trouble with __hot on inlines. So you would always need to mark the caller.
-Andi
-- ak@linux.intel.com -- Speaking for myself only.
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