Bug 82681 - gcc warning on possible code problem is issuedinconstently
gcc warning on possible code problem is issuedinconstently
Product: Red Hat Linux
Classification: Retired
Component: gcc3 (Show other bugs)
i386 Linux
medium Severity medium
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Assigned To: Jakub Jelinek
Depends On:
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Reported: 2003-01-24 15:51 EST by Michael Marxmeier
Modified: 2008-05-01 11:38 EDT (History)
0 users

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Doc Type: Bug Fix
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Last Closed: 2003-01-24 15:54:11 EST
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Description Michael Marxmeier 2003-01-24 15:51:27 EST
From Bugzilla Helper:
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.3b) Gecko/20030119

Description of problem:
Two problems with gcc-3.2-7

1. gcc miscompiles my code (which might be a bordercase
   but is accepted with all other compilers i've tested

2. gcc issues a warning in non-optimizing mode but not 
   when -O is present.

Stupid example is below ...

--- a.c ---
#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>

int main(int argc, char *argv[])
        char *p;

        p = argv[1];
                *p++ = toupper(*p);
        printf("%s\n", argv[1]);
        return 0;
--- end a.c ---

$ cc -Wall a.c
a.c: In function `main':
a.c:11: warning: operation on `p' may be undefined
$ ./a.out hallo
$ cc -Wall -O a.c
./a.out hallo

Version-Release number of selected component (if applicable):

How reproducible:

Steps to Reproduce:
see above

Additional info:
Comment 1 Jakub Jelinek 2003-01-24 15:54:11 EST
Your code triggers undefined behaviour, and as such gcc can do anything it wants
with it.
Comment 2 Michael Marxmeier 2003-01-24 16:35:40 EST
As i wrote above, it might be undefined behavior and 
i'm not arguing that (it's just likely to break lots
of existing code).

However (IMHO) gcc should issue the warning consistently
if -Wall is present. I've seen a number of cases where the 
warning was not issued, with -O used.

I'm not pushing this but please consider to reopen the bug
for the issue #2 (warning) - i've updated the subject

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