[Bug target/17390] missing floating point compare optimization

[uros at kss-loka dot si]

------- Comment #8 from uros at kss-loka dot si  2006-01-18 09:50 -------
(In reply to comment #7)

> Hmm, I get (but that looks like different branch predictions):

It looks that your default is -mtune=pentium.

> _testf:
>         fldl    4(%esp)
>         ftst
>         fnstsw  %ax
>         testb   $64, %ah
>         jne     L10
>         ftst
>         fnstsw  %ax
>         fstp    %st(0)
>         testb   $69, %ah
>         jne     L5
>         fld1
>         ret
>         .align 2,0x90
> L10:
>         fstp    %st(0)
>         fldz
>         ret
> L5:
>         flds    LC2
>         ret

With proposed patch, this code is compiled to (-O2 -ffast-math -mtune=pentium
-fomit-frame-pointer):

testf:
        fldl   4(%esp)
        ftst
        fnstsw %ax
        fstp   %st(0)
        testb  $64, %ah
        jne .L10
        testb  $69, %ah
        jne .L5
        fld1
        ret
        .p2align 4,,7
.L10:
        fldz
        ret
.L5:
        flds   .LC2
        ret

and for -mtune=i686:

testf:
        fldl   4(%esp)
        ftst
        fnstsw %ax
        fstp   %st(0)
        sahf
        je .L10
        jbe .L5
        fld1
        ret
        .p2align 4,,7
.L10:
        fldz
        .p2align 4,,8
        ret
.L5:
        flds   .LC2
        .p2align 4,,4
        ret

BTW: I'll attach a patch, rediffed to current SVN.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17390