[Bug tree-optimization/105835] [13 Regression] Dead Code Elimination Regression at -O1 (trunk vs. 12.1.0)

cvs-commit at gcc dot gnu.org gcc-bugzilla@gcc.gnu.org
Sat Jun 18 08:10:26 GMT 2022


--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Roger Sayle <sayle@gcc.gnu.org>:


commit r13-1162-g9991d84d2a84355fd3fc9afc89a963f45991bfa9
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Sat Jun 18 09:06:20 2022 +0100

    PR tree-optimization/105835: Two narrowing patterns for match.pd.

    This patch resolves PR tree-optimization/105835, which is a code quality
    (dead code elimination) regression at -O1 triggered/exposed by a recent
    change to canonicalize X&-Y as X*Y.  The new (shorter) form exposes some
    missed optimization opportunities that can be handled by adding some
    extra simplifications to match.pd.

    One transformation is to simplify "(short)(x ? 65535 : 0)" into the
    equivalent "x ? -1 : 0", or more accurately x ? (short)-1 : (short)0",
    as INTEGER_CSTs record their type, and integer conversions can be
    pushed inside COND_EXPRs reducing the number of gimple statements.

    The other transformation is that (short)(X * 65535), where X is [0,1],
    into the equivalent (short)X * -1, (or again (short)-1 where tree's
    INTEGER_CSTs encode their type).  This is valid because multiplications
    where one operand is [0,1] are guaranteed not to overflow, and hence
    integer conversions can also be pushed inside these multiplications.

    These narrowing conversion optimizations can be identified by range
    analyses, such as EVRP, but these are only performed at -O2 and above,
    which is why this regression is only visible with -O1.

    2022-06-18  Roger Sayle  <roger@nextmovesoftware.com>

            PR tree-optimization/105835
            * match.pd (convert (mult zero_one_valued_p@1 INTEGER_CST@2)):
            Narrow integer multiplication by a zero_one_valued_p operand.
            (convert (cond @1 INTEGER_CST@2 INTEGER_CST@3)): Push integer
            conversions inside COND_EXPR where both data operands are
            integer constants.

            PR tree-optimization/105835
            * gcc.dg/pr105835.c: New test case.

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