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Date: Fri, 13 Nov 2015 10:03:00 -0000
Message-ID: <CAFULd4b-NbcO9PcpKgw0PtJ2jaAFy4f1jdxLZ2erNc6MHsbUOg@mail.gmail.com>
Subject: Re: [PATCH 1/2] simplify-rtx: Simplify trunc of and of shiftrt
From: Uros Bizjak <ubizjak@gmail.com>
To: "gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>
Cc: Segher Boessenkool <segher@kernel.crashing.org>
Content-Type: text/plain; charset=UTF-8
X-SW-Source: 2015-11/txt/msg01635.txt.bz2

Hello!

> 2015-11-09  Segher Boessenkool  <segher@kernel.crashing.org>
>
> * gcc/simplify-rtx.c (simplify_truncation): Simplify TRUNCATE
> of AND of [LA]SHIFTRT.

Revision r230164 (the above patch) regressed:

FAIL: gcc.target/alpha/pr42269-1.c scan-assembler-not addl

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]))))

Uros.