On Fri, Jan 11, 2013 at 12:46 PM, Václav Zeman <vhaisman@xxxxxxxxx> wrote:
>
> I am wondering if there are not some improvements that could be made in
> generation of x86 FPU code. Here is a simple sign function:
>
> float signf4(float x)
> {
> return x < 0.0f
> ? -1.0f
> : 1.0f;
> }
>
> It generates the following assembler code (GCC 4.7.2, g++ -m32 -O3
> -fverbose-asm -save-temps -g3 -ggdb -march=native):
>
> _Z6signf4f:
> .LFB84:
> .loc 1 27 0
> .cfi_startproc
> .LVL4:
> .loc 1 30 0
> fld1
> fldz
> flds 4(%esp) # x
> fxch %st(1) # ??? Why?
> fucomip %st(1), %st #,
> ffreep %st(0) #
> fld1
> fchs
> fcmovbe %st(1), %st #,,
> fstp %st(1) #
> .loc 1 31 0
> ret
>
> I am wondering why is the fxch instruction necessary and why is the code
> not instead like this?
>
> _Z6signf4f:
> .LFB84:
> .loc 1 27 0
> .cfi_startproc
> .LVL4:
> .loc 1 30 0
> fld1
> flds 4(%esp) # ??? Load the parameter before the zero.
> fldz # ??? to avoid the fxch instruction.
> fucomip %st(1), %st #,
> [...]
I don't know. Please open a bug report as described at
http://gcc.gnu.org/bugs . Thanks.
Note that the 387 floating point operations are not high priority
these days, as it is generally better to use SSE.
Ian
