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From: Eric Botcazou <botcazou@adacore.com>
X-Google-Original-From: Eric Botcazou <ebotcazou@adacore.com>
To: Claudiu Zissulescu Ianculescu <claziss@gmail.com>
Cc: gcc@gcc.gnu.org
Subject: Re: Possible regression in DF analysis
Date: Thu, 08 Dec 2022 12:33:12 +0100
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> 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