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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103388] [12 Regression] missed optimization
 for dead code elimination at -O3 (trunk vs 11.2.0)
Date: Tue, 18 Jan 2022 14:24:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103388

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2021-11-23 00:00:00         |2022-1-18
             Status|NEW                         |ASSIGNED

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
Re-confirmed.(In reply to Jeffrey A. Law from comment #3)
> So to fix this right we'd need to duplicate some of the logic in
> tree-ssa-threadupdate.c.  Conceptually for block B where one or more
> predecessors thread to target T, you make a single copy B', and redirect
> *all* the relevant predecessors to B'.
>=20
> In addition to allowing more aggressive threading, it would also reduce
> codesize since currently we'll end up with multiple copies of B'.  We have
> optimizers that are supposed to clean that up, but I've never seen them d=
o a
> particularly good job.
>=20
> This isn't likely to land in gcc-12.
>=20
> An interim approach might be to go ahead and register the thread and only
> reject it for size later if we're going to end up with multiple copies.=20
> After all this is a cost analysis question and we don't know until all the
> paths are registered if it's profitable or not.

So I think at least this should be possible, no?  Also why do we need to
do extra limitation?  We should end up accounting for B's size N times
without the optimization so the costing is still accurate, no?

So IMHO the scaling factors do not make much sense to me, they were introdu=
ced
to fix PR68398.

We need --param fsm-scale-path-stmts=3D1 to get the desired threading done.=