[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

--- Comment #6 from Uroš Bizjak <ubizjak at gmail dot com> ---
On a related note, gcc-5 (-O2) was able to also simplify:

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

        cmplt $16,$17,$17
        s4addl $17,$18,$0
        ret $31,($26),1

where gcc-8 emits:

        cmplt $16,$17,$0
        s4addq $0,0,$0
        addl $0,$18,$0
        ret $31,($26),1

gcc-5 is able to if-convert to:

(insn 30 10 15 2 (set (reg:DI 70 [ D.1498 ])
        (if_then_else:DI (ne (reg:DI 76)
                (const_int 0 [0]))
            (const_int 4 [0x4])
            (const_int 0 [0]))) cmp1.c:5 161 {*movdicc_internal}
     (nil))

which is later combined with:

(insn 15 30 16 2 (set (reg:SI 77 [ D.1498 ])
        (plus:SI (subreg/s/u:SI (reg:DI 70 [ D.1498 ]) 0)
            (subreg/s/u:SI (reg/v:DI 75 [ c ]) 0))) cmp1.c:5 3 {addsi3}
     (expr_list:REG_DEAD (reg/v:DI 75 [ c ])
        (expr_list:REG_DEAD (reg:DI 70 [ D.1498 ])
            (nil))))

to:

Trying 30 -> 15:
Successfully matched this instruction:
(set (reg:SI 77 [ D.1498 ])
    (plus:SI (mult:SI (subreg:SI (reg:DI 76) 0)
            (const_int 4 [0x4]))
        (reg:SI 18 $18 [ c ])))

gcc-8 is not able to if-convert...