From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-368097-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 5734 invoked by alias); 18 Sep 2011 13:23:48 -0000
Received: (qmail 5717 invoked by uid 22791); 18 Sep 2011 13:23:46 -0000
X-SWARE-Spam-Status: No, hits=-2.8 required=5.0	tests=ALL_TRUSTED,AWL,BAYES_00,TW_RJ
X-Spam-Check-By: sourceware.org
Received: from localhost (HELO gcc.gnu.org) (127.0.0.1)    by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Sun, 18 Sep 2011 13:23:33 +0000
From: "gjl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/50448] New: [avr] Missed optimization accessing struct component with known, absolute address.
Date: Sun, 18 Sep 2011 13:38:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: gjl at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Changed-Fields:
Message-ID: <bug-50448-4@http.gcc.gnu.org/bugzilla/>
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
Content-Type: text/plain; charset="UTF-8"
MIME-Version: 1.0
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-bugs.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-help@gcc.gnu.org>
Sender: gcc-bugs-owner@gcc.gnu.org
X-SW-Source: 2011-09/txt/msg01274.txt.bz2

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50448

             Bug #: 50448
           Summary: [avr] Missed optimization accessing struct component
                    with known, absolute address.
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: gjl@gcc.gnu.org
                CC: eric.weddington@atmel.com
            Target: avr


typedef struct
{
    unsigned char a,b,c,d;
} SPI_t;

#define SPIE (*(SPI_t volatile*) 0x0AC0)

void foo (void)
{
    SPIE.d = 0xAA;
    while (!(SPIE.c & 0x80));

    SPIE.d = 0xBB;
    while (!(SPIE.c & 0x80));
}

avr-gcc-4.6.1 -Os -S -fdump-tree-optimized -fdump-rtl-expand 
compiles that code to

foo:
    ldi r24,lo8(-86)
    ldi r30,lo8(2752)
    ldi r31,hi8(2752)
    std Z+3,r24
.L2:
    lds r24,2754
    sbrs r24,7
    rjmp .L2
    ldi r24,lo8(-69)
    ldi r30,lo8(2752)
    ldi r31,hi8(2752)
    std Z+3,r24
.L3:
    lds r24,2754
    sbrs r24,7
    rjmp .L3
    ret

Instead of loading the address 2752 two times, it's sufficient to load it once
or to do a direct access to 2755 and avoid loading the constant altogether.

The load appeard first in .expand; .optimized looks fine:

foo ()
{
  signed char D.1932;
  volatile unsigned char D.1931;
  signed char D.1930;
  volatile unsigned char D.1929;

<bb 2>:
  MEM[(volatile struct SPI_t *)2752B].d ={v} 170;

<bb 3>:
  D.1929_3 ={v} MEM[(volatile struct SPI_t *)2752B].c;
  D.1930_4 = (signed char) D.1929_3;
  if (D.1930_4 >= 0)
    goto <bb 3>;
  else
    goto <bb 4>;

<bb 4>:
  MEM[(volatile struct SPI_t *)2752B].d ={v} 187;

<bb 5>:
  D.1931_7 ={v} MEM[(volatile struct SPI_t *)2752B].c;
  D.1932_8 = (signed char) D.1931_7;
  if (D.1932_8 >= 0)
    goto <bb 5>;
  else
    goto <bb 6>;

<bb 6>:
  return;
}