[Bug rtl-optimization/58517] icvt (after combine) puts ccreg clobbering insn between ccset insn and ccreg use

[olegendo at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58517

Oleg Endo <olegendo at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |steven at gcc dot gnu.org

--- Comment #3 from Oleg Endo <olegendo at gcc dot gnu.org> ---
The problematic transformation happens in ifcvt.c (noce_try_cmove_arith).
I've only briefly looked over that code and can think of the following options
how to fix the issue:

1) Check whether insn_a and insn_b modify the src reg (ccreg) of the
conditional branch.  If they do, don't do the transformation.
On SH, the resulting code would be the same as not specifying -mpretend-cmove:

        cmp/ge  r5,r4
        mov     r6,r0
        bf/s    .L6
        sub     r4,r0
        rts
        nop
.L6:
        sett
        mov     r5,r0
        rts
        subc    r4,r0

2) Check whether insn_a and insn_b modify the src reg (ccreg) of the
conditional branch.  If they do, try to move the insn that sets the ccreg
before the conditional branch, _after_ the emitted insn_a and insn_b and right
before the conditional move.
On SH, the resulting code would look something like this:

        sett
        mov     r5,r0
        sub     r4,r6
        subc    r4,r0
        cmp/ge  r5,r4
        bf      0f
        mov     r6,r0
0:
        rts
        nop

Steven, since you're RTL optimizers reviewer, could you please provide some
feedback regarding the matter?