On 11/09/12 11:25, Christophe Lyon wrote:
> On 10 September 2012 19:30, Richard Earnshaw <rearn...@arm.com> wrote:
>> On 10/09/12 16:40, Christophe Lyon wrote:
>>> Why do we have to keep room for the predicate here? (%?) Doesn't this
>>> pattern match only in unconditional cases?
>>>
>>
>> Because the ARM back-end has a very late conditionalizer pass that can
>> also generate conditional execution. It very rarely kicks in these
>> days, but if the predication rules are in there you could end up with an
>> instruction that the compiler thought was conditionally executed being
>> always run. That would be bad^TM.
>>
>
> Thanks for the clarification.
>
>>> BTW, I didn't manage to have GCC generate conditional revsh. I merely
>>> added an "if (y)" guard before calling builtin_bswap16, but this
>>> didn't turn into a conditional revsh.
>>>
> On this topic, could you suggest a way to generate conditional revsh?
>
> I would like to augment the testsuite for this, and I tried:
>
> int y;
> short swaps16(short x) {
> if (y)
> return __builtin_bswap16(x);
> }
> but it generates:
> swaps16:
> @ args = 0, pretend = 0, frame = 0
> @ frame_needed = 0, uses_anonymous_args = 0
> @ link register save eliminated.
> movw r3, #:lower16:y @ 50 *arm_movsi_vfp/4 [length = 4]
> movt r3, #:upper16:y @ 51 *arm_movt [length = 4]
> ldr r3, [r3] @ 7 *arm_movsi_vfp/5 [length = 4]
> cmp r3, #0 @ 8 *arm_cmpsi_insn/3 [length = 4]
> beq .L3 @ 9 arm_cond_branch [length = 4]
> revsh r0, r0 @ 13 *arm_revsh/3 [length = 4]
> bx lr @ 56 *arm_return [length = 12]
> .L3:
> bx lr @ 58 *arm_return [length = 12]
>
> ie unconditional revsh.
>
>
> Another question regarding the *arm_revsh pattern you wrote: why is
> the "arch" set to "t1,t2,32" ? Shouldn't it be "t1,t2,a" ?
> (IIUC, "32" matches both "a" and "t2" as per the definition of TARGET_32BIT)
>
> Thanks
>
> Christophe.
>
Try something like:
short foo(int);
short swaps (short x, int y)
{
int z = x;
if (y)
z = __builtin_bswap16(x);
return foo (z);
}
If that's not enough, try adding 1 to z before calling foo.
R.
