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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/103006] [9/10/11/12 Regression] wrong code at
 -O1 or -O2 on x86_64-linux-gnu by r7-7101
Date: Wed, 02 Feb 2022 11:44:44 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103006

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org,
                   |                            |rsandifo at gcc dot gnu.org

--- Comment #13 from Richard Biener <rguenth at gcc dot gnu.org> ---
So I have a patch that adds explicit birth markers (using clobbers specially
marked).  That works well sofar but it conflicts with clobbers (not marked =
as
birth) that are added for clobbering at the start of variable lifetime like
C++ does at the beginning of CTORs.  I for example see

  inst =3D{v} {CLOBBER(birth)};
  inst =3D{v} {CLOBBER};  (**)
  inst.v =3D 42;
...
  inst =3D{v} {CLOBBER};

where (**) is inserted by the C++ frontend (with -flifetime-dse which is
the default).  Indeed my life analysis for the purpose of stack slot
sharing now only relies on the birth/death markers so it gets confused
by the extra clobber.

We now also have some use-after-free diagnostic that would likely trip
over this as it assumes that a CLOBBER ends lifetime of storage.

I guess disentangling both use-cases by also marking the
end-of-storage-lifetime
clobbers specially would solve both issues.=