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

[Paolo Bonzini <paolo dot bonzini at lu dot unisi dot ch>]

> It will take 2 days. SPEC CPU 2K results aren't very encouraging.
> One benchmark is down by 7% on IA32.  I suspect AMD processors may
> have similar issues. Given the SPEC CPU 2K/2006 impact I have been
> seeing on IA32 and Intel64, and the current addresing model has been
> used for such a long time, I don't think it is a good idea to make
> such a change which affects every architecture for one architecture.

Some SPEC benchmarks are notoriously volatile, and the more 
sophisticated the cores, the worse this can be.  The impact of this 
patch has been analyzed a handful of times.  Once it was an effect of 
loop alignment, arguably a latent bug.  This time it was out-of-order 
execution, and nobody outside Intel knows how this works.  Next time we 
don't know what it will be.

I understand your position because in the beginning I was also thinking 
that this change should have been done in target-dependent code; however 
1) the benefits on POWER have been clearly identified, unlike the reason 
for the performance problems on Intel 2) canonicalization is in general 
a good thing, and 3) the next iteration of Intel/AMD processors might 
also prefer base registers to come first in indexed addressing modes.

Paolo