Hi!
On Fri, Nov 13, 2015 at 11:02:55AM +0100, Uros Bizjak wrote:
> on alpha-linux-gnu.
>
> The difference starts in combine, where before the patch, we were able
> to combine insns:
>
> (insn 7 6 8 2 (set (reg:DI 82)
> (lshiftrt:DI (reg:DI 81 [ x ])
> (const_int 16 [0x10]))) pr42269-1.c:8 66 {lshrdi3}
> (expr_list:REG_DEAD (reg:DI 81 [ x ])
> (nil)))
> (insn 8 7 11 2 (set (reg:DI 70 [ _2 ])
> (sign_extend:DI (subreg:SI (reg:DI 82) 0))) pr42269-1.c:8 2
> {*extendsidi2_1}
> (expr_list:REG_DEAD (reg:DI 82)
> (nil)))
>
> to:
>
> Trying 7 -> 8:
> Successfully matched this instruction:
> (set (reg:DI 70 [ _2 ])
> (zero_extract:DI (reg/v:DI 80 [ x ])
> (const_int 16 [0x10])
> (const_int 16 [0x10])))
> allowing combination of insns 7 and 8
> original costs 4 + 4 = 8
> replacement cost 4
> deferring deletion of insn with uid = 7.
> modifying insn i3 8: r70:DI=zero_extract(r80:DI,0x10,0x10)
> deferring rescan insn with uid = 8.
>
> After the patch, the combination fails:
>
> Trying 7 -> 8:
> Failed to match this instruction:
> (set (reg:DI 70 [ _2 ])
> (sign_extend:DI (lshiftrt:SI (subreg:SI (reg/v:DI 80 [ x ]) 0)
> (const_int 16 [0x10]))))
Somehow, before the patch, it decided to do a zero-extension (where the
combined insns had a sign extension). Was that even correct? Maybe
many bits of reg 80 (or, hrm, 81 in the orig?!) are known zero?
Segher
