From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-205472-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 13786 invoked by alias); 20 Dec 2006 14:35:50 -0000
Received: (qmail 13760 invoked by uid 48); 20 Dec 2006 14:35:40 -0000
Date: Wed, 20 Dec 2006 14:35:00 -0000
Subject: [Bug c/30267]  New: missed optimization due to bad range propagation without -fwrapv
X-Bugzilla-Reason: CC
Message-ID: <bug-30267-128@http.gcc.gnu.org/bugzilla/>
Reply-To: gcc-bugzilla@gcc.gnu.org
To: gcc-bugs@gcc.gnu.org
From: "bruno at clisp dot org" <gcc-bugzilla@gcc.gnu.org>
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: 2006-12/txt/msg01657.txt.bz2

This program shows that some range propagation became worse between
gcc 4.0.2 and gcc 4.1.1.

=========================== foo.c ========================
int notneg (int x)
{
  return (~ -x) >= (-2147483647-1);
}
int negnot (int x)
{
  return (- ~x) <= 2147483647;
}
==========================================================


# With gcc 4.0.2 on i686-pc-linux-gnu the code is fully optimized:

$ gcc -O2 -fomit-frame-pointer -S foo.c && cat foo.s
        .file   "foo.c"
        .text
        .p2align 4,,15
.globl notneg
        .type   notneg, @function
notneg:
        movl    $1, %eax
        ret
        .size   notneg, .-notneg
        .p2align 4,,15
.globl negnot
        .type   negnot, @function
negnot:
        movl    $1, %eax
        ret
        .size   negnot, .-negnot
        .ident  "GCC: (GNU) 4.0.2"
        .section        .note.GNU-stack,"",@progbits


# With gcc 4.1.1 on i686-pc-linux-gnu the code is fully optimized with -fwrapv
# but not without -fwrapv:

$ gcc -O2 -fomit-frame-pointer -S foo.c && cat foo.s
        .file   "foo.c"
        .text
        .p2align 4,,15
.globl notneg
        .type   notneg, @function
notneg:
        xorl    %eax, %eax
        cmpl    $-2147483648, 4(%esp)
        setne   %al
        ret
        .size   notneg, .-notneg
        .p2align 4,,15
.globl negnot
        .type   negnot, @function
negnot:
        xorl    %eax, %eax
        cmpl    $2147483647, 4(%esp)
        setne   %al
        ret
        .size   negnot, .-negnot
        .ident  "GCC: (GNU) 4.1.1"
        .section        .note.GNU-stack,"",@progbits

$ gcc -O2 -fomit-frame-pointer -fwrapv -S foo.c && cat foo.s
        .file   "foo.c"
        .text
        .p2align 4,,15
.globl notneg
        .type   notneg, @function
notneg:
        movl    $1, %eax
        ret
        .size   notneg, .-notneg
        .p2align 4,,15
.globl negnot
        .type   negnot, @function
negnot:
        movl    $1, %eax
        ret
        .size   negnot, .-negnot
        .ident  "GCC: (GNU) 4.1.1"
        .section        .note.GNU-stack,"",@progbits

So somehow this seems to be linked to flag_wrapv. But regardless which
value is the result after signed overflow, any int >= INT_MIN and
any int <= INT_MAX should evaluate to 1 unconditionally.


-- 
           Summary: missed optimization due to bad range propagation without
                    -fwrapv
           Product: gcc
           Version: 4.1.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bruno at clisp dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30267