Re: Possible regression in DF analysis

[Claudiu Zissulescu Ianculescu <claziss@gmail.com>]

Hi Eric,
The problem shows in loop-doloop.c when I introduce a loop end pattern
that replaces the first jump instruction (JUMP_INSN 15) with a pattern
that clobbers CC reg. However, the DF doesn't look like it works as
the doloop step cannot find the CC reg alive. Please see
loop-doloop.c:766. Hence, it introduces the doloop_end patterns, and
renders the compare instruction (INSN 14) dead code. leading to
errors.

Thanks,
Claudiu

On Thu, Dec 8, 2022 at 1:33 PM Eric Botcazou <botcazou@adacore.com> wrote:
>
> > However, after this optimization I get the CC reg being dead after
> > JUMP_INSN 15, which may lead to wrong code gen. Here it is the dump
> > from fwprop1:
> >
> > (insn 14 11 15 3 (set (reg:CC 66 cc)
> >         (compare:CC (reg/v:SI 98 [ bytes ])
> >             (const_int 8 [0x8]))) "bad_cc.c":11:8 406 {cmpsi}
> >      (nil))
> > (jump_insn 15 14 16 3 (set (pc)
> >         (if_then_else (gtu (reg:CC 66 cc)
> >                 (const_int 0 [0]))
> >             (label_ref 27)
> >             (pc))) "bad_cc.c":11:8 15 {condjump}
> >      (expr_list:REG_DEAD (reg:CC 66 cc)
> >         (int_list:REG_BR_PROB 955630228 (nil)))
> >  -> 27)
> > (note 16 15 18 4 [bb 4] NOTE_INSN_BASIC_BLOCK)
> > (jump_insn 18 16 19 4 (set (pc)
> >         (if_then_else (eq (reg:CC 66 cc)
> >                 (const_int 0 [0]))
> >             (label_ref:DI 34)
> >             (pc))) "bad_cc.c":12:10 15 {condjump}
> >      (expr_list:REG_DEAD (reg:CC 66 cc)
> >         (int_list:REG_BR_PROB 365072228 (nil)))
> >  -> 34)
> >
> > Please observe the REG_DEAD note on the both jump instructions.
>
> Passes that consume REG_DEAD/REG_UNUSED notes need to recompute them
> explicitly, they are not updated on the fly.
>
> --
> Eric Botcazou
>
>