https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105231
--- Comment #22 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 13 Apr 2022, ebotcazou at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105231
> 
> --- Comment #21 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> > I'm not sure that's a general enough fix though since we seem to drop
> > the REG_EQUAL note and as soon as we do that there's a disconnect
> > between what CFG generation thinks throws and what combine thinks
> > is good enough to put a REG_EH_REGION note on.
> 
> Yes, we actually drop 2 REG_EQUAL notes:
> 
> (insn 22 17 78 3 (set (reg:SF 92)
>         (mem/u/c:SF (symbol_ref/u:SI ("*.LC0") [flags 0x2]) [0  S4 A32]))
> "pr105231.c":6:3 142 {*movsf_internal}
>      (expr_list:REG_EQUAL (const_double:SF 1.8446744073709551616e+19
> [0x0.8p+65])
>         (nil)))
> 
> (insn 78 22 24 3 (set (reg:XF 99)
>         (float_extend:XF (reg:SF 92))) 166 {*extendsfxf2_i387}
>      (expr_list:REG_DEAD (reg:SF 92)
>         (expr_list:REG_EQUAL (const_double:XF 1.8446744073709551616e+19
> [0x0.8p+65])
>             (nil))))
> 
> -->
> 
> (insn 78 22 24 3 (set (reg:XF 99)
>         (float_extend:XF (mem/u/c:SF (symbol_ref/u:SI ("*.LC0") [flags 0x2]) 
> [0
>  S4 A32]))) 166 {*extendsfxf2_i387}
>      (expr_list:REG_EH_REGION (const_int 1 [0x1])
>         (nil)))
> 
> so an immediate solution would be to preserve the one on insn 78 (and arrange
> for the purge_dead_edge magic to trigger again).

Yes.  If i3 is always the insn we combine into any REG_EQUAL note
on that insn can always prevail.  I suppose the purge_dead_edge magic
would need to be postponed until we distributed all notes - not sure
where a safe place would be to remove EH_REGION notes in combine
after note distribution finalized?

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