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Re: [PING][PATCH][REVISED] Fix PR middle-end/PR28690, modify swap_commutative_operands_p


On Tue, Jun 26, 2007 at 09:09:47PM -0500, Peter Bergner wrote:
> On Tue, 2007-06-26 at 16:27 -0700, Ian Lance Taylor wrote:
> > Peter's patch referenced above looks OK to me, and I'm inclined to
> > approve it.  But some of these numbers do look troubling.  Why do we
> > get such large swings in some of the benchmarks?
> 
> Well, we mentioned one reason we saw in:
> 
>   http://gcc.gnu.org/ml/gcc-patches/2007-06/msg00808.html
> 
> which was loop alignment related when x86_64's ".p2align 4,,7" isn't
> able to align the loop to what we asked for.  In that case, it's just
> luck or bad luck on what alignment we end up with.  In this case, it
> looks like HJ used his r125920 patch which should have helped some here,
> but given his change only allows skipping up to 10 bytes now, there
> might still be cases where we don't get the alignment we want.

That is not true:

http://gcc.gnu.org/ml/gcc-cvs/2007-06/msg00684.html

My patch adds ".p2align 3" after ".p2align 4,,X" for x86-64.

> 
> I'm not sure about x86/x86_64 hardware, but on POWER, two identical
> loops that have the same alignment in the lowest nibble of their
> addresses can still have performance differences depending on where
> they show up in the cache line/cache line sector.
> 
> HJ, can you send the binaries for 436.cactusADM, 454.calculix and
> 200.sixtrack to Pat and I and we'll try and track down exactly why
> they're slowing down when he gets back from vacation later this week.

Sure.


H.J.


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