From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id 303B3389246B; Mon, 29 Jun 2020 12:49:24 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 303B3389246B
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1593434964;
 bh=JjQU+bWBwoH/ROTrVbvFQwMkUAiQepyGknJjGx0r+wo=;
 h=From:To:Subject:Date:From;
 b=m8XWDez8SvAnAQx2/3ZdMLPkItrXBdc2vDvaPkz82R6hFE/zXZnfsaYXQxYAfea3z
 2FnkTTiO065kh0PXPJ3HMcHaF1pUBjsWn3PNuSqiheqrs47ArRLg+Ul6sG4Grsthjj
 gfsKHc8FQfcqNVm9NVx3eyE0cBbNGrtlZ8uVdg84=
From: "rsandifo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/95962] New: Inefficient code for simple arm_neon.h iota
 operation
Date: Mon, 29 Jun 2020 12:49:23 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: 11.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: rsandifo at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 keywords bug_severity priority component assigned_to reporter blocked
 target_milestone cf_gcctarget
Message-ID: <bug-95962-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <http://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <http://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Mon, 29 Jun 2020 12:49:24 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95962

            Bug ID: 95962
           Summary: Inefficient code for simple arm_neon.h iota operation
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rsandifo at gcc dot gnu.org
            Blocks: 95958
  Target Milestone: ---
            Target: aarch64*-*-*

For:

#include <arm_neon.h>

int32x4_t
foo (void)
{
  int32_t array[] =3D { 0, 1, 2, 3 };
  return vld1q_s32 (array);
}

we produce:

foo:
.LFB4217:
        .cfi_startproc
        sub     sp, sp, #16
        .cfi_def_cfa_offset 16
        mov     x0, 2
        mov     x1, 4294967296
        movk    x0, 0x3, lsl 32
        stp     x1, x0, [sp]
        ldr     q0, [sp]
        add     sp, sp, 16
        .cfi_def_cfa_offset 0
        ret

In contrast, clang produces essentially perfect code:

        adrp    x8, .LCPI0_0
        ldr     q0, [x8, :lo12:.LCPI0_0]
        ret

I think the problem is a combination of two things:

- __builtin_aarch64_ld1v4si & co. are treated as general
  functions rather than pure functions, so in principle
  it could write to the given address.  This stops us
  promoting the array to a constant.

- The loads could be reduced to native gimple-level
  operations, at least on little-endian targets.

IMO this a bug rather than an enhancement.  Intrinsics only
exist to optimise code, and what GCC is doing falls short
of what users should reasonably expect.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95958
[Bug 95958] [meta-bug] Inefficient arm_neon.h code for AArch64=