[Bug rtl-optimization/47979] New: Problem in comparing integers

[Bug rtl-optimization/47979] New: Problem in comparing integers

[xiaofengguo at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47979

           Summary: Problem in comparing integers
           Product: gcc
           Version: 4.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: xiaofengguo@google.com


Hi,

Here is a short summary of the problem. In the upgrading of gcc from 3.3 to
4.4.3, my hash unit test failed. I did some investigation and wrote a simple
program, which is attached, to reproduce this problem.

I use command below to compile this program:
g++ -g -pipe -O9 -funroll-loops -ffast-math -DNDEBUG -march=pentiumpro -DLINUX
-fpic -DGEOTARGETING -D_NG_POSIX_THREAD -D_RWCONFIG_${RWDBGNUM}_$RWLIBVERSION
-DRW_MULTI_THREAD -DRW_NO_XMSG -DRW_POSIX_D10_THREADS -D_REENTRANT -Wall
-Werror -Wno-sign-compare -Wno-write-strings -Wno-strict-aliasing
-Wno-unused-result abs_error.cc

and the result shows me "hash<0" is false. If "-O9" is removed from the command
line, "hash<0" is true.

I disassembly the obj file by objdump (objdump -Sl a.out), and below is digest
of the optimized result:

...
 80484f1:       b8 49 5e 86 da          mov    $0xda865e49,%eax
 80484f6:       89 e5                   mov    %esp,%ebp
 80484f8:       83 e4 f0                and    $0xfffffff0,%esp
 80484fb:       53                      push   %ebx
 80484fc:       e8 4c 00 00 00          call   804854d <__i686.get_pc_thunk.bx>
 8048501:       81 c3 f3 1a 00 00       add    $0x1af3,%ebx
 8048507:       83 ec 1c                sub    $0x1c,%esp
 804850a:       89 44 24 08             mov    %eax,0x8(%esp)
 804850e:       c7 04 24 01 00 00 00    movl   $0x1,(%esp)
 8048515:       8d 8b ec e6 ff ff       lea    -0x1914(%ebx),%ecx
 804851b:       89 4c 24 04             mov    %ecx,0x4(%esp)
 804851f:       e8 dc fe ff ff          call   8048400 <__printf_chk@plt>
 8048524:       8d 83 f7 e6 ff ff       lea    -0x1909(%ebx),%eax
 804852a:       ba 49 5e 86 da          mov    $0xda865e49,%edx
 804852f:       89 54 24 08             mov    %edx,0x8(%esp)
 8048533:       89 44 24 04             mov    %eax,0x4(%esp)
 8048537:       c7 04 24 01 00 00 00    movl   $0x1,(%esp)
 804853e:       e8 bd fe ff ff          call   8048400 <__printf_chk@plt>
...

Seems "hash" and "result" both assigned 0xda865e49 (-628728247 in oct)
directly, for the optimizations.

I am not sure whether it is a problem in optimizations of the compiler, so I am
not sure whether it is correct for me to assign bug on this component. And,
because I am not familiar with i86 assembly, just read some tutorial to
understand a bit of the asm code, there must be some problems in the
investigations. Please let me know what do you think of this issue. Sure, if it
is my problem or there is a duplication of this bug, please let me know and
close the bug freely.

And, because it is almost impossible for our project to move back to gcc 3.3 or
try one more upgrade in short period, would you share me some way for us to
walk around this problem? (Sure, to make sure other code won't face this
problem any more)

Many thanks for your help!

[Bug rtl-optimization/47979] Problem in comparing integers

[xiaofengguo at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47979

--- Comment #1 from Xiaofeng Guo <xiaofengguo at google dot com> 2011-03-03 20:36:32 UTC ---
Because I can't find the attachment in the thread, add the text below for
debugging easily.

==================================================
#include <stdio.h>
#include <string.h>

int main() {
  const char *str = "1234567";

  int hash = 17;
  for (int i = 0; i < strlen(str); ++i) {
    hash = 37 * hash + str[i];
  }
  printf("hash = %d, %d\n", hash, hash < 0);
  int result = (hash < 0) ? (-hash) : hash;
  printf("result = %d\n", result);
  return 0;
}
===================================================

[Bug rtl-optimization/47979] Problem in comparing integers

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47979

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

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

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-03-03 20:37:56 UTC ---
overflow for signed integer is undefined.  Use unsigned integers if you want
defined wrapping behavior.