------- Additional Comments From paolo dot bonzini at lu dot unisi dot ch
2005-09-21 06:51 -------
Subject: Re: x87 reg allocated for constants for -mfpmath=sse
>Note that in this pattern cost computation of MMX_REGS are all ignored ('*'
>in front of y). So, the cost
>which is computed is for 'r' which is GENERAL_REGS. This cost is too high and
>eventually results in
>memory cost to be lower than register cost. I tried the following simple patch
>as experiment and got all
>the performance back (it is now comparable with 4.0). Note that in this patch,
>I removed the '*' in the
>2nd alternative so cost of keeping the operand in mmx_regs class is factored
>in. This resulted in a
>lower cost than that of memory. Is this the way to go? This is just an
>experiment which seems to work.
>
>
I think it makes sense. The x86 back-end is playing too many tricks
(such as the # classes) with the register allocator and regclass
especially, and they are biting back.
Still, I'd rather hear from an expert as to why the classes were written
like this.
Paolo
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19653
