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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102880] [12 Regression] Dead Code Elimination
 Regression at -O3
Date: Fri, 29 Oct 2021 09:09:48 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102880

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Created attachment 51699
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D51699&action=3Dedit
hack to make forwarders

This is an overly simple attempt to make forwarders which works to restore =
the
DCE.

Given this it probably makes sense to at least move the early merge_phi pass
after CD-DCE.  That doesn't help this testcase since regular CFG cleanup
performs the merging in our case.  It's remove_forwarder_block will perform
this.

An alternative band-aid would be to resolve the regression by not running
CFG cleanup after DSE (we're failing to run it when we empty a BB as well,
something we might want to fix instead).

Undoing PHI merging might be also a good thing to perform before going out =
of
SSA.  And merge_phi itself might do this as a 2nd step.=