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 7C0C23857353; Thu, 26 May 2022 00:49:50 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 7C0C23857353
From: "jrtc27 at jrtc27 dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/105733] New: riscv: Poor codegen for large stack frames
Date: Thu, 26 May 2022 00:49:50 +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: 12.1.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: jrtc27 at jrtc27 dot com
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
 bug_severity priority component assigned_to reporter target_milestone
 cf_gcctarget
Message-ID: <bug-105733-4@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: <https://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: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Thu, 26 May 2022 00:49:50 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105733

            Bug ID: 105733
           Summary: riscv: Poor codegen for large stack frames
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jrtc27 at jrtc27 dot com
  Target Milestone: ---
            Target: riscv*-*-*

For the following test:

#define BUF_SIZE 2064

void
foo(unsigned long i)
{
    volatile char buf[BUF_SIZE];

    buf[i] =3D 0;
}

GCC currently generates:

foo:
        li      t0,-4096
        addi    t0,t0,2016
        li      a4,4096
        add     sp,sp,t0
        li      a5,-4096
        addi    a4,a4,-2032
        add     a4,a4,a5
        addi    a5,sp,16
        add     a5,a4,a5
        add     a0,a5,a0
        li      t0,4096
        sd      a5,8(sp)
        sb      zero,2032(a0)
        addi    t0,t0,-2016
        add     sp,sp,t0
        jr      ra

whereas Clang generates the much shorter:

foo:
        lui     a1, 1
        addiw   a1, a1, -2016
        sub     sp, sp, a1
        addi    a1, sp, 16
        add     a0, a0, a1
        sb      zero, 0(a0)
        lui     a0, 1
        addiw   a0, a0, -2016
        add     sp, sp, a0
        ret

The:

        li      a4,4096
        ...
        li      a5,-4096
        addi    a4,a4,-2032
        add     a4,a4,a5

sequence in particular is rather surprising to see rather than just li a4,-=
2032
and constant-folding that would halve the instruction count difference betw=
een
GCC and Clang alone.

See: https://godbolt.org/z/8EGc85dsf=