Re: [PATCH] New *truncsfdf2_i387_1 pattern for i386.md

[Uros Bizjak <uros at kss-loka dot si>]

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.