[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
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           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