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From: "krebbel at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/101523] Huge number of combine attempts
Date: Tue, 20 Jul 2021 09:41:38 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101523

--- Comment #1 from Andreas Krebbel <krebbel at gcc dot gnu.org> ---
This appears to be triggered by try_combine unnecessarily setting back the
position by returning the i2 insn.

When 866 is inserted into 973 866 still needs to be kept around for other
users. So try_combine first merges the two sets into a parallel and immedia=
tely
notices that this can't be recognized. Because none of the sets is a trivial
move it is split again into two separate insns. Although the new i2 pattern
exactly matches the input i2 combine considers this to be a new insn and
triggers all the scanning log link creation and eventually returns it what
let's the combine start all over at 866.

Due to that combine tries many of the substitutions more than 400x.

Trying 866 -> 973:
  866: r22393:DI=3Dr22391:DI+r22392:DI
  973: r22499:DF=3Dr22498:DF*[r22393:DI]
      REG_DEAD r22498:DF
Failed to match this instruction:
(parallel [
        (set (reg:DF 22499)
            (mult:DF (reg:DF 22498)
                (mem:DF (plus:DI (reg/f:DI 22391 [ _85085 ])
                        (reg:DI 22392 [ _85086 ])) [17 *_85087+0 S8 A64])))
        (set (reg/f:DI 22393 [ _85087 ])
            (plus:DI (reg/f:DI 22391 [ _85085 ])
                (reg:DI 22392 [ _85086 ])))
    ])
Failed to match this instruction:
(parallel [
        (set (reg:DF 22499)
            (mult:DF (reg:DF 22498)
                (mem:DF (plus:DI (reg/f:DI 22391 [ _85085 ])
                        (reg:DI 22392 [ _85086 ])) [17 *_85087+0 S8 A64])))
        (set (reg/f:DI 22393 [ _85087 ])
            (plus:DI (reg/f:DI 22391 [ _85085 ])
                (reg:DI 22392 [ _85086 ])))
    ])
Successfully matched this instruction:
(set (reg/f:DI 22393 [ _85087 ])
    (plus:DI (reg/f:DI 22391 [ _85085 ])
        (reg:DI 22392 [ _85086 ])))
Successfully matched this instruction:
(set (reg:DF 22499)
    (mult:DF (reg:DF 22498)
        (mem:DF (plus:DI (reg/f:DI 22391 [ _85085 ])
                (reg:DI 22392 [ _85086 ])) [17 *_85087+0 S8 A64])))
allowing combination of insns 866 and 973
original costs 4 + 4 =3D 8
replacement costs 4 + 4 =3D 8
modifying insn i2   866: r22393:DI=3Dr22391:DI+r22392:DI
deferring rescan insn with uid =3D 866.
modifying insn i3   973: r22499:DF=3Dr22498:DF*[r22391:DI+r22392:DI]
      REG_DEAD r22498:DF
deferring rescan insn with uid =3D 973.=