[Bug tree-optimization/94782] New: Simple multiplication-related arithmetic not optimized to direct multiplication

[Bug tree-optimization/94782] New: Simple multiplication-related arithmetic not optimized to direct multiplication

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94782

            Bug ID: 94782
           Summary: Simple multiplication-related arithmetic not optimized
                    to direct multiplication
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

int f(int a, int b)
{
    return (int)((a - 1U) * b) + b;
}

Can be optimized to `a * b`. LLVM does this transformation, GCC does not.

Comparison here : https://godbolt.org/z/LFJwFJ

[Bug tree-optimization/94782] Simple multiplication-related arithmetic not optimized to direct multiplication

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94782

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2020-04-27

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Since the inner (a - 1U) * b is unsigned but a * b would be signed due to
undefined signed overflow we cannot optimize to that.  But we could indeed.
optimize to (unsigned)a * (unsigned)b.  fold-const.c contains related
transforms that could be amended.  reassoc could as well but would need
enhancement for signed arithmetic.

[Bug tree-optimization/94782] Simple multiplication-related arithmetic not optimized to direct multiplication

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94782

--- Comment #2 from Gabriel Ravier <gabravier at gmail dot com> ---
Appears to be fixed on trunk.

[Bug tree-optimization/94782] Simple multiplication-related arithmetic not optimized to direct multiplication

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94782

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |TREE

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
It is fixed on the RTL level but not the gimple level.


Combine does it for x86_64:
Trying 7, 8 -> 9:
    7: {r91:SI=r93:SI-0x1;clobber flags:CC;}
      REG_DEAD r93:SI
      REG_UNUSED flags:CC
    8: {r92:SI=r91:SI*r94:SI;clobber flags:CC;}
      REG_UNUSED flags:CC
      REG_DEAD r91:SI
    9: r90:SI=r92:SI+r94:SI
      REG_DEAD r94:SI
      REG_DEAD r92:SI
Successfully matched this instruction:
(set (reg:SI 90)
    (mult:SI (reg:SI 93)
        (reg:SI 94)))
allowing combination of insns 7, 8 and 9
original costs 4 + 12 + 4 = 20
replacement cost 12

But it fails to do it on aarch64:

Trying 7 -> 14:
    7: r101:SI=r103:SI-0x1
      REG_DEAD r103:SI
   14: x0:SI=r101:SI*r104:SI+r104:SI
      REG_DEAD r101:SI
      REG_DEAD r104:SI

[Bug tree-optimization/94782] Simple multiplication-related arithmetic not optimized to direct multiplication

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94782

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The RTL level for x86_64 was fixed with
r11-6456-g4615cde5d7ef281d4b554df411f82ad707f0a54d (aka PR 98334).