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Date: Wed, 31 Mar 2021 10:14:26 +0200
From: Jan Hubicka <hubicka@ucw.cz>
To: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
Cc: gcc-bugs@gcc.gnu.org
Subject: Re: [Bug ipa/99835] missed optimization for dead code elimination at
 -O3 (vs. -O1)
Message-ID: <20210331081426.GD14482@kam.mff.cuni.cz>
References: <bug-99835-4@http.gcc.gnu.org/bugzilla/>
 <bug-99835-4-mo2KzlvFbG@http.gcc.gnu.org/bugzilla/>
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> At -O3 the unused 'c' remains.  Likely different (recursive?) inlining makes us
> process a cgraph cycle in different order and thus fail to elide the output
> of 'c' (it's output first at -O3).
> 
> Fixing that would need processing cgraph SCCs with an extra IPA phase in main
> optimization so we get a chance to do extra node removal (maybe order
> the cycles so that functions we can elide - aka static ones - are processed
> last).
That would tamper with optimizations that propagate from callee to
caller during late optimization, like IPA register allocation, stack
alignment propagation or late pure/const discovery.

Honza