Eliminated function return values in retarget
Hi folks,
I'm retargetting GCC to a custom architecture
(https://github.com/jamieiles/gcc-oldland is the retarget) and have an
odd problem with function return values near the end of a function being
eliminated. For example:
extern int getreturn(void);
int foo(void)
{
if (getreturn() != 0)
return -1;
return 0;
}
I see that with optimization (-O1 or greater) that this gets compiled
into a call to getreturn() and then unconditionally sets the return
value to -1 (with a one_cmpl + eor + ior). Inserting a call to another
function inbetween the 'return -1' and 'return 0' is enough to cause
working code to be generated.
works fine at -O0 generating
:
0: f0 ce ff 93 str32 $lr, [$sp, 0x1ffc]
4: d0 8e ff 93 str32 $fp, [$sp, 0x1ff8]
8: 0d 8e 00 08 sub $fp, $sp, 0x8
c: 0e 8e 00 08 sub $sp, $sp, 0x8
10: fb ff ff 40 call0
10: R_OLDLAND_PC24 getreturn-0x4
14: 01 00 00 3e mov $r1, $r0
18: 00 00 00 3c mov $r0, 0x0
1c: 00 01 00 32 cmp $r1, $r0
20: 03 00 00 58 beq 30
24: 00 fc ff ef movhi $r0, 0x
28: 00 fc ff f7 orlo$r0, $r0, 0x
2c: 01 00 00 50 b 34
30: 00 00 00 3c mov $r0, 0x0
34: 0e 8e 00 00 add $sp, $sp, 0x8
38: 0d 8e 00 00 add $fp, $sp, 0x8
3c: 0d 8e ff 83 ldr32 $fp, [$sp, 0x1ff8]
40: 0f ce ff 83 ldr32 $lr, [$sp, 0x1ffc]
44: f0 00 00 46 ret
but at -O1 I get
:
0: f0 ce ff 93 str32 $lr, [$sp, 0x1ffc]
4: d0 8e ff 93 str32 $fp, [$sp, 0x1ff8]
8: 0d 8e 00 08 sub $fp, $sp, 0x8
c: 0e 8e 00 08 sub $sp, $sp, 0x8
10: fb ff ff 40 call0
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
24: 0e 8e 00 00 add $sp, $sp, 0x8
28: 0d 8e 00 00 add $fp, $sp, 0x8
2c: 0d 8e ff 83 ldr32 $fp, [$sp, 0x1ff8]
30: 0f ce ff 83 ldr32 $lr, [$sp, 0x1ffc]
34: f0 00 00 46 ret
I have $r0-$r5 as call used registers and my TARGET_FUNCTION_VALUE and
TARGET_FUNCTION_VALUE_REGNO_P using $r0.
Any pointers would be greatfully received.
Regards,
Jamie
Re: Eliminated function return values in retarget
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 call0
> > 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
