[Bug c/40189] New: gcc 4.4.0 incorrectly folds add-as-comparison

[Bug c/40189] New: gcc 4.4.0 incorrectly folds add-as-comparison

[mat at lcs dot mit dot edu]

This code:

int
foo(int arg0)
{
  if ((-2147483647 - 1) + arg0)
    return 20;
  else
    return 11;
}

when compiled with gcc-4.4.0 at -O2 or higher, simply returns 20
unconditionally:

$ ~/x86-gcc4/bin/gcc -m32 -O2 -S ~/baz.c -o /dev/tty
        .file   "baz.c"
        .text
        .p2align 4,,15
.globl foo
        .type   foo, @function
foo:
        pushl   %ebp
        movl    $20, %eax
        movl    %esp, %ebp
        popl    %ebp
        ret
        .size   foo, .-foo
        .ident  "GCC: (GNU) 4.4.0"
        .section        .note.GNU-stack,"",@progbits


This bug appears to be architecture-independent (I've seen it on x86,
x86_64, and another chip).

The bug did not happen in gcc-4.0.2.

Here is some information about my system:

$ ~/x86-gcc4/bin/gcc --version
gcc (GCC) 4.4.0
Copyright (C) 2009 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ uname -a
Linux ld-1 2.6.9-42.0.8.ELsmp #1 SMP Tue Jan 30 12:18:01 EST 2007 x86_64 x86_64
x86_64 GNU/Linux

$ file bin/gcc
bin/gcc: ELF 64-bit LSB executable, AMD x86-64, version 1 (SYSV), for GNU/Linux
2.4.0, dynamically linked (uses shared libs), not stripped


-- 
           Summary: gcc 4.4.0 incorrectly folds add-as-comparison
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mat at lcs dot mit dot edu
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40189

[Bug c/40189] gcc 4.4.0 incorrectly folds add-as-comparison

[schwab at linux-m68k dot org]

------- Comment #1 from schwab at linux-m68k dot org  2009-05-18 19:47 -------
Not a bug.  If arg0 is negative you get undefined behavior due to overflow. 
Use -fwrapv to get wrapping semantics.


-- 

schwab at linux-m68k dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40189

[Bug c/40189] gcc 4.4.0 incorrectly folds add-as-comparison

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2009-05-18 19:48 -------
INT_MIN + any number is never going to be zero because INT_MIN = - INT_MAX -1.

Also signed integer overflow is undefined which gives that the above.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40189

[Bug c/40189] gcc 4.4.0 incorrectly folds add-as-comparison

[mat at lcs dot mit dot edu]

------- Comment #3 from mat at lcs dot mit dot edu  2009-05-18 20:53 -------
Fair enough, thanks (this came up running a compiler test suite that checks
even undefined edge cases).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40189