[PATCH] New *truncsfdf2_i387_1 pattern for i386.md

[Uros Bizjak]

Hello Roger!

>With the patch below we now generate:
>
>foo:    pushl   %edx
>        fldl    d
>        fstps   f
>        popl    %eax
>        ret
>
>We still (unfortunately) allocate the stack slot, but combine.c is
>now able to recognize the result of merging the float_truncate with
>the write to memory.  Interestingly, we already have the equivalent
>*truncxfsf2_387_1 and *truncxfdf2_387_1 patterns, so its a surprise
>that the df->sf pattern is missing, but I suspect this is due to the
>labyrinthine interactions between x87 and sse2 math.  To play it safe,
>the new pattern is only enabled if we know we can't use sse2, but
>that might be overly conservative.  Uros or RTH?
>
>  
>
It is the job of "*truncdfsf_fast_i387" to handle writes to memory. 
However, this pattern is at the moment enabled only for 
flag_unsafe_math_optimizations. If your example is compiled with 
additional -ffast-math flag, the result is indeed the same as with your 
patch.

To fix this oversight, I suggest to change the constraint of 
"*truncdfsf_fast_i387" into:

  "TARGET_80387 && (MEM_P (operands[0]) || flag_unsafe_math_optimizations)"

to  acheive the same functionality as with your proposed patch.

Thanks,
Uros.