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From: "jgreenhalgh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/65235] Simplifying vec_select of vec_concat miscompiles when first element of vec_concat is const_int
Date: Mon, 02 Mar 2015 11:06:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65235

James Greenhalgh <jgreenhalgh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|aarch64                     |aarch64*-*-*
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-03-02
                 CC|                            |jgreenhalgh at gcc dot gnu.org
               Host|                            |*-*-*
     Ever confirmed|0                           |1

--- Comment #7 from James Greenhalgh <jgreenhalgh at gcc dot gnu.org> ---
The tescase from comment 5 fails for me at -O3 on an aarch64-none-elf cross
compiler (r221086), this also fails for native aarch64-none-linux-gnu.

As with Kyrill, I see:

(insn 16 15 17 2 (set (reg:V2DI 80 [ D.2809 ])
        (vec_concat:V2DI (reg:DI 95 [ D.2812 ])
            (reg:DI 77 [ D.2806 ]))) reduced.c:19 1386
{aarch64_combine_internaldi}
     (expr_list:REG_DEAD (reg:DI 95 [ D.2812 ])
        (expr_list:REG_DEAD (reg:DI 77 [ D.2806 ])
            (expr_list:REG_EQUAL (vec_concat:V2DI (reg:DI 95 [ D.2812 ])
                    (const_int 4503599627370498 [0x10000000000002]))
                (nil)))))

(insn 17 16 18 2 (set (reg:DI 96 [ D.2806 ])
        (vec_select:DI (reg:V2DI 80 [ D.2809 ])
            (parallel [
                    (const_int 0 [0])
                ]))) reduced.c:14 1372 {aarch64_get_lanev2di}
     (expr_list:REG_DEAD (reg:V2DI 80 [ D.2809 ])
        (nil)))
----
Combine:

Trying 16 -> 17:
Successfully matched this instruction:
(set (reg:DI 96 [ D.2806 ])
    (reg:DI 77 [ D.2806 ]))
---

Which is bogus, the correct rewrite would be to:

(set (reg:DI 96 [ D.2806 ])
    (reg:DI 95 [ D.2812 ]))