Should I file a bug report? If it is not a C semantics thing, GCC certainly
produces unnecessarily big code.
.file "tst.c"
.text
.p2align 4,,15
.globl tst1
.type tst1, @function
tst1:
.LFB0:
.cfi_startproc
movswl %si,%esi
movswl %di,%edi
xorl %eax, %eax
subl $254, %esi
cmpl %edi, %esi
setg %al
ret
.cfi_endproc
.LFE0:
.size tst1, .-tst1
.p2align 4,,15
.globl tst2
.type tst2, @function
tst2:
.LFB1:
.cfi_startproc
subw $255, %si
xorl %eax, %eax
cmpw %di, %si
setge %al
ret
.cfi_endproc
.LFE1:
.size tst2, .-tst2
.ident "GCC: (GNU) 4.4.0 20090218 (experimental) [trunk revision
143368]"
.section .note.GNU-stack,"",@progbits
> -----Original Message-----
> From: Richard Guenther [mailto:richard.guent...@gmail.com]
> Sent: 03 March 2009 15:16
> To: Bingfeng Mei
> Cc: gcc@gcc.gnu.org; John Redford
> Subject: Re: Why are these two functions compiled differently?
>
> On Tue, Mar 3, 2009 at 4:06 PM, Bingfeng Mei
> <b...@broadcom.com> wrote:
> > Hello,
> > I came across the following example and their
> .final_cleanup files. To me, both functions should produce
> the same code. But tst1 function actually requires two extra
> sign_extend instructions compared with tst2. Is this a C
> semantics thing, or GCC mis-compile (over-conservatively) in
> the first case.
>
> Both transformations are already done by the fronted (or fold), likely
> shorten_compare is quilty for tst1 and fold_unary for tst2 (which
> folds (short)((int)b - (int)A).
>
> Richard.
>
> > Cheers,
> > Bingfeng Mei
> > Broadcom UK
> >
> >
> > #define A 255
> >
> > int tst1(short a, short b){
> > if(a > (b - A))
> > return 0;
> > else
> > return 1;
> >
> > }
> >
> >
> > int tst2(short a, short b){
> > short c = b - A;
> > if(a > c)
> > return 0;
> > else
> > return 1;
> >
> > }
> >
> >
> > .final_cleanup
> > ;; Function tst1 (tst1)
> >
> > tst1 (short int a, short int b)
> > {
> > <bb 2>:
> > return (int) b + -254 > (int) a;
> >
> > }
> >
> >
> >
> > ;; Function tst2 (tst2)
> >
> > tst2 (short int a, short int b)
> > {
> > <bb 2>:
> > return (short int) ((short unsigned int) b + 65281) >= a;
> >
> > }
> >
> >
> >
> >
>
>
