[Bug target/53976] [SH] Unnecessary clrt after bt

["olegendo at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53976

--- Comment #1 from Oleg Endo <olegendo at gcc dot gnu.org> 2012-09-23 20:50:23 UTC ---
The clrt insn gets placed into another basic block, thus using a peephole will
not work in this case.  In order to be able to eliminate the clrt (or any sett)
the value of the T bit must be tracked not only inside a basic block but also
across basic blocks.

Another case, which shows that the T bit value is lost and has to be
recalculated:

int test_2 (volatile int* a, int b, int c)
{
  a[1] = b != 0;

  if (b == 0)
    a[10] = c;

  return b == 0;
}

compiled with -O2 -m4:
        tst     r5,r5
        mov     #-1,r1
        negc    r1,r1
        mov.l   r1,@(4,r4)
        tst     r5,r5        !! OK, negc above clobbers T bit
        bf      .L2
        mov.l   r6,@(40,r4)
.L2:
        tst     r5,r5        !! T bit lost in new BB
        rts
        movt    r0


compiled with -O2 -m2a:
        tst     r5,r5
        movrt   r1
        mov.l   r1,@(4,r4)
        bf.s    .L4
        tst     r5,r5           !! T bit lost in new BB
    mov.l    r6,@(40,r4)
        tst     r5,r5           !! T bit lost in new BB
.L4:
        rts
        movt    r0