RFC - COST of const_double for x86 prevents constant copy propagation in cse

[Fariborz Jahanian]

On Aug 25, 2005, at 12:47 PM, H. J. Lu wrote:

> On Thu, Aug 25, 2005 at 12:37:32PM -0700, Ian Lance Taylor wrote:
>
>> Fariborz Jahanian <fjahanian@apple.com> writes:
>>
>>
>>> Forgot to attach the patch:
>>>
>>> Index: i386.c
>>> ===================================================================
>>> RCS file: /cvs/gcc/gcc/gcc/config/i386/i386.c,v
>>> retrieving revision 1.795.4.33
>>> diff -c -p -r1.795.4.33 i386.c
>>> *** i386.c      15 Aug 2005 23:36:10 -0000      1.795.4.33
>>> --- i386.c      25 Aug 2005 17:08:33 -0000
>>> *************** ix86_rtx_costs (rtx x, int code, int out
>>> *** 15730,15740 ****
>>>          else
>>>          switch (standard_80387_constant_p (x))
>>>            {
>>> !         case 1: /* 0.0 */
>>> !           *total = 1;
>>> !           break;
>>> !         default: /* Other constants */
>>> !           *total = 2;
>>>              break;
>>>            case 0:
>>>            case -1:
>>> --- 15730,15737 ----
>>>          else
>>>          switch (standard_80387_constant_p (x))
>>>            {
>>> !         default: /* All constants */
>>> !           *total = 0;
>>>              break;
>>>            case 0:
>>>            case -1:
>>>
>>
>> For what it's worth, as I told Fariborz, I suspect that returning  
>> 0 is
>> correct for SFmode, but I'm somewhat doubtful for DFmode.  And his
>> test case is odd since the resulting code has more instructions  
>> and is
>> larger.  I know little about x86 instruction timings, but it seems
>> surprising that the new sequence is faster.  Maybe the problem is in
>> using %xmm0 instead of one of the 80387 registers--or, since this is
>> after all merely a constant--one of the general registers.
>>
>> And in any case this type of thing should be controlled by an  
>> entry in
>> the i386 processor_costs structure.
>>
>
> I think the problem may be somewhere else. I got the same xmm0 code
> sequence on Linux/ia32 with -msse3 -mfpmath=sse. However, I got
>
>         xorl    %eax, %eax
>         movq    %rax, 16(%rdi)
>         movq    %rax, 8(%rdi)
>         movq    %rax, (%rdi)

Can you try this with -march=pentium4

- fariborz

>
> on Linux/x86-64.
>
>
> H.J.
>