From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-447320-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 29962 invoked by alias); 24 Mar 2014 20:59:13 -0000
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
Received: (qmail 29935 invoked by uid 48); 24 Mar 2014 20:59:09 -0000
From: "akruppa at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/60641] New: Converting ushort to offset on x86_64 generates double movzwl
Date: Mon, 24 Mar 2014 20:59:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 4.7.3
X-Bugzilla-Keywords:
X-Bugzilla-Severity: normal
X-Bugzilla-Who: akruppa at gmail dot com
X-Bugzilla-Status: UNCONFIRMED
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
Message-ID: <bug-60641-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-SW-Source: 2014-03/txt/msg02189.txt.bz2

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60641

            Bug ID: 60641
           Summary: Converting ushort to offset on x86_64 generates double
                    movzwl
           Product: gcc
           Version: 4.7.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: akruppa at gmail dot com

The following test cases produce sub-optimal assembly output.
This was verified with these three versions of gcc:
gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
gcc (SUSE Linux) 4.7.3
gcc (Debian 4.8.2-16) 4.8.2


#include <stdlib.h>
unsigned short
foo (const unsigned short *start, const unsigned char *mask)
{
   unsigned short r = 0;
   unsigned short ux = *start;
   if (mask[ux])
     r = ux;
   return r;
}
void bar(unsigned int *s, unsigned short a)
{
  s[a] = a;
}


Compile with, e.g., 

gcc -std=c99 -g -W -Wall -O3 -c movzwl.c

The effect also occurs when using -O2 instead.

The foo() function contains:
   0x0000000000000000 <+0>:     movzwl (%rdi),%edx
   0x0000000000000003 <+3>:     xor    %eax,%eax
   0x0000000000000005 <+5>:     movzwl %dx,%ecx
   0x0000000000000008 <+8>:     cmpb   $0x0,(%rsi,%rcx,1)


The foo() function is:
   0x0000000000000010 <+0>:     movzwl %si,%eax
   0x0000000000000013 <+3>:     movzwl %si,%esi
   0x0000000000000016 <+6>:     mov    %esi,(%rdi,%rax,4)
   0x0000000000000019 <+9>:     retq   


In both cases, an unnecessary movzwl instruction is generated. This may be the
same issue as #36873, which was rejected because the test cases used volatile
accesses. These test cases here show that the duplicate movzwl occurs without
volatile as well.