[Bug target/19653] x87 reg allocated for constants for -mfpmath=sse

["paolo dot bonzini at lu dot unisi dot ch" <gcc-bugzilla@gcc.gnu.org>]

------- 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


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19653