[Bug rtl-optimization/83628] [8 Regression] performance regression when accessing arrays on alpha

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83628

Uroš Bizjak <ubizjak at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
                 CC|                            |law at gcc dot gnu.org,
                   |                            |rsandifo at gcc dot gnu.org,
                   |                            |segher at gcc dot gnu.org
          Component|target                      |rtl-optimization
           Assignee|ubizjak at gmail dot com           |unassigned at gcc dot gnu.org
   Target Milestone|---                         |8.0
            Summary|performance regression when |[8 Regression] performance
                   |accessing arrays on alpha   |regression when accessing
                   |                            |arrays on alpha

--- Comment #4 from Uroš Bizjak <ubizjak at gmail dot com> ---
The remaining regression is in make_compound_operation function in combine.c.

In the following testcase:

int test (int a, int b)
{
  return a + b * 4;
}

the compiler (gcc-5) combined:

(insn 7 4 8 2 (set (reg:DI 76)
        (ashift:DI (reg/v:DI 74 [ b ])
            (const_int 2 [0x2]))) cmp.c:3 64 {ashldi3}
     (expr_list:REG_DEAD (reg/v:DI 74 [ b ])
        (nil)))
(insn 8 7 9 2 (set (reg:SI 75 [ D.1494 ])
        (plus:SI (subreg:SI (reg:DI 76) 0)
            (subreg/s/u:SI (reg/v:DI 73 [ a ]) 0))) cmp.c:3 3 {addsi3}
     (expr_list:REG_DEAD (reg:DI 76)
        (expr_list:REG_DEAD (reg/v:DI 73 [ a ])
            (nil))))
into:

Trying 7 -> 8:
Successfully matched this instruction:
(set (reg:SI 75 [ D.1494 ])
    (plus:SI (mult:SI (reg:SI 17 $17 [ b ])
            (const_int 4 [0x4]))
        (reg:SI 16 $16 [ a ])))

This is not the case anymore, the compiler (gcc-8) fails combination with:

Trying 7 -> 8:
    7: r75:DI=$17:DI<<0x2
      REG_DEAD $17:DI
    8: r74:SI=$16:SI+r75:DI#0
      REG_DEAD $16:DI
      REG_DEAD r75:DI
Failed to match this instruction:
(set (reg:SI 74)
    (plus:SI (subreg:SI (ashift:DI (reg:DI 17 $17 [ b ])
                (const_int 2 [0x2])) 0)
        (reg:SI 16 $16 [ a ])))

This is a rtl-optimization regression from gcc-5 branch.