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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/98334] Failure to optimally optimize add loop
 to mul
Date: Thu, 17 Dec 2020 10:04:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98334

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This boils down to:
int
bar (int x, int y)
{
  return (int) (y - 1U) * x + x;
}

int
baz (int x, int y)
{
  return (y - 1) * x + x;
}

We do optimize the latter, but don't optimize the former into y * x.
For non-wrapping operation in bar that would be an invalid optimization,
as (int) (y - 1U) * x + x is well defined for y =3D=3D INT_MIN and x =3D=3D=
 -1:
it is INT_MAX * -1 + -1, i.e. INT_MIN without overflow, but INT_MIN * -1
does overflow.
But we perhaps could and should turn that into just (int) ((y - 1U) * x),
i.e. unsigned multiplication.
We shouldn't do this until very late though, because turning signed arithme=
tics
into unsigned may disable other optimizations as it has no UB.

Or we could optimize this on RTL, combiner attempts:
Trying 7, 8 -> 9:
    7: {r90:SI=3Dr93:SI-0x1;clobber flags:CC;}
      REG_DEAD r93:SI
      REG_UNUSED flags:CC
    8: {r91:SI=3Dr90:SI*r92:SI;clobber flags:CC;}
      REG_UNUSED flags:CC
      REG_DEAD r90:SI
    9: r89:SI=3Dr91:SI+r92:SI
      REG_DEAD r92:SI
      REG_DEAD r91:SI
Failed to match this instruction:
(set (reg:SI 89)
    (plus:SI (mult:SI (plus:SI (reg:SI 93)
                (const_int -1 [0xffffffffffffffff]))
            (reg:SI 92))
        (reg:SI 92)))
so if we hack up simplify-rtx.c to optimize that to (mult:SI (reg:SI 93)
(reg:SI 92)), it should work.=