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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/108949] New: Optimize shift counts
Date: Mon, 27 Feb 2023 12:36:00 +0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108949

            Bug ID: 108949
           Summary: Optimize shift counts
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

>From https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108941#c13 :
Because various backends support shift count truncation or have patterns th=
at
recognize it in certain cases, I wonder if middle-end couldn't canonicalize
shift count (N + x)
where N is multiple of shift first operand's bitsize B to x & (B - 1) where=
 the
latter
is often optimized away while the former is not.
For similar N - x it is more questionable because N - x is a single GIMPLE
statement while -y & (B - 1) are two; perhaps it could be done at expansion
time though.
In generic code at least for SHIFT_COUNT_TRUNCATED targets, otherwise maybe=
 if
one can easily detect negation optab and subtraction instruction not accept=
ing
immediate for the minuend.  Or handle all this in each of the backends?

int
foo (int x, int y)
{
  return x << (y & 31);
}

int
bar (int x, int y)
{
  return x << (32 + y);
}

int
baz (int x, int y)
{
  return x << (-y & 31);
}

int
qux (int x, int y)
{
  return x << (32 - y);
}=