http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53976
--- Comment #3 from Oleg Endo <olegendo at gcc dot gnu.org> ---
(In reply to Oleg Endo from comment #2)
> Interestingly, the following function shows some improved behavior (notice
> the removed volatile mem store):
>
> int test_2_1 (int* a, int b, int c)
> {
> a[1] = b != 0;
>
> if (b == 0)
> a[10] = c;
>
> return b == 0;
> }
>
> -O2 -m2a:
> tst r5,r5
> movrt r1
> mov.l r1,@(4,r4)
> bf .L4
> mov.l r6,@(40,r4)
> .L4:
> rts
> movt r0
>
>
> This is already minimal.
> However, for non-SH2A it's still the same:
> tst r5,r5
> mov #-1,r1
> negc r1,r1
> tst r5,r5
> bf/s .L4
> mov.l r1,@(4,r4)
> mov.l r6,@(40,r4)
> tst r5,r5
> .L4:
> rts
> movt r0
One of the problems in this case is that negc clobbers the T bit. Another
alternative
movt r0
xor #1,r0
should be selected here. This could be done by looking at the insns around the
negc-movrt and check whether some insn after negc-movrt sets the T bit in the
same way as it was set before the negc-movrt. In this case not clobbering the
T bit would eliminate the redundant test. However, if this pattern occurs in a
loop or pressure on R0 is high, using negc and the redundant test is probably
going to be better.
