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From: "law at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/102981] [12 Regression] Dead Code Elimination
 Regression at -O3 (trunk vs 11.2.0)
Date: Thu, 04 Nov 2021 18:47:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102981

Jeffrey A. Law <law at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

--- Comment #4 from Jeffrey A. Law <law at gcc dot gnu.org> ---
I have no strong opinions about this specific testcase.  More generally I a=
m in
agreement with Zdenek and others that the threaders should not be peeling
iterations off loops or rotating loops.

Fundamentally the threaders don't have the kind of costing model to know if
peeling an iteration off is profitable or not.  So even after the loop
optimizers are done, I'd still lean against peeling since if it was profita=
ble
it should have been done by the loop optimizer or vectorizer.

So unless someone can show this is a significant issue in real world code, I
would  argue that it ought to be fixed by including the possibility of
eliminating unreachable code int he profitibility analysis for loop peeling=
 by
the loop optimizers and possibly the unroller (for this specific example).=