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From: "antoshkka at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/114270] New: Integer multiplication on floating
 point constant with conversion back to integer is not optimized
Date: Thu, 07 Mar 2024 15:31:55 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: middle-end
X-Bugzilla-Version: 14.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: antoshkka at gmail dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114270

            Bug ID: 114270
           Summary: Integer multiplication on floating point constant with
                    conversion back to integer is not optimized
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

Consider the following example:

unsigned test(unsigned x) {
    return (unsigned)(x * 0.5);
}

With -O2 GCC generates the code with a fair conversion to fp and
multiplication:

test(unsigned int):
  mov edi, edi
  pxor xmm0, xmm0
  cvtsi2sd xmm0, rdi
  mulsd xmm0, QWORD PTR .LC0[rip]
  cvttsd2si rax, xmm0
  ret

However the multiplication does not overflow and the floating point constan=
t is
a normal number.

A more optimal code should look like the following:

test(unsigned int):
  mov eax, edi
  shr eax
  ret

Probably the optimization could be used for
* any multiplication of integer on positive fp-number less or equal to 1.0
* any division of integer on positive fp-number greater or equal to 1.0
if the result is converted back to integer=