Hi Michael,
On Tue, Oct 14, 2014 at 04:24:13PM +0200, Michael Matz wrote:
> Hi,
>
> On Tue, 14 Oct 2014, Jamie Iles wrote:
>
> > int foo(void)
> > {
> > if (getreturn() != 0)
> > return -1;
> >
> > return 0;
> > }
>
> So if getreturn() returns zero it can simply reuse that return value ...
>
> > but at -O1 I get
> >
> > 10: fb ff ff 40 call 0 <foo>
> > 10: R_OLDLAND_PC24 getreturn-0x4
> > 14: 01 00 00 3c mov $r1, 0x0
> > 18: 01 01 00 0a sub $r1, $r1, $r0
> > 1c: 00 01 00 2a or $r0, $r1, $r0
> > 20: 00 f0 01 38 asr $r0, $r0, 0x1f
>
> ... and if I'm interpreting the mnemonics correctly that is what seems to
> happen here. If $r0 is zero, then:
>
> mov $r1, 0x0 $r1 = 0
> sub $r1, $r1, $r0 $r1 = 0 - 0 = 0
> or $r0, $r1, $r0 $r0 = 0 | 0 = 0
> asr $r0, $r0, 0x1f $r0 = 0 >> 31 = 0
>
> Voila, zero is correctly returned. Any non-zero value will be transformed
> into -1 (if positive the high bit will be set due to subtraction, if
> negative the high bit will be set due to the 'or', and the shift
> replicates the high bit into the lower ones, yielding -1).
Thanks, this definitely put me in the right direction, I'd been staring
at it for too long! It turns out that the problem was that I'd
previously been testing with one_cmplsi2 defined which *was* wrong and I
thought I'd reduced it to this. It turns out that the actual problem
was that I had one_cmplsi2 defined with
(define_insn "one_cmplsi2"
[(set (match_operand:SI 0 "register_operand" "=r")
(neg:SI (match_operand:SI 1 "register_operand" "r")))]
""
"xor %0, %1, -1")
and had operand 1 as neg:SI rather than not:SI. Fixing that gives me
the correct code at all optimization levels.
Thanks again for looking at this, much appreciated!
Jamie
