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Date: Fri, 13 Nov 2015 14:36:00 -0000
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Uros Bizjak <ubizjak@gmail.com>
Cc: "gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH 1/2] simplify-rtx: Simplify trunc of and of shiftrt
Message-ID: <20151113143649.GA27800@gate.crashing.org>
References: <CAFULd4b-NbcO9PcpKgw0PtJ2jaAFy4f1jdxLZ2erNc6MHsbUOg@mail.gmail.com>
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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