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List:       gcc-bugs
Subject:    optimization/4751: g++ -O producing incorrect code for ((global1&255)==255 || global2)
From:       keenan () ghs ! com
Date:       2001-10-30 23:36:15
[Download RAW message or body]


> Number:         4751
> Category:       optimization
> Synopsis:       g++ -O producing incorrect code for ((global1&255)==255 || global2)
> Confidential:   no
> Severity:       serious
> Priority:       medium
> Responsible:    unassigned
> State:          open
> Class:          wrong-code
> Submitter-Id:   net
> Arrival-Date:   Tue Oct 30 15:46:00 PST 2001
> Closed-Date:
> Last-Modified:
> Originator:     keenan@ghs.com
> Release:        gcc version 3.0.1 (also reproduced on 2.95.2 and 2.95.3)
> Organization:
> Environment:
HP-UX 11.0 (also reproduced on 10.20)
> Description:
An illegal optimization is being performed, such that the attached file produces \
incorrect output when compiled with -O (or -O1, -O2, -O3). The simple test case \
breaks down to code that is of the form (where both globals are type int)

if(((global1&255)==global1) || global2)
    exectute_some_statement();

The statement is _always_ executed, even when both expressions are false. If split \
into two tests: if(((global1&255)==global1))
    exectute_some_statement();
if(global2)
    exectute_some_statement();

The statment is not executed.

I wasn't able to reproduce this bug on non-HP-UX hosts.
> How-To-Repeat:
g++ -O foo.cc
./a.out
> Fix:

> Release-Note:
> Audit-Trail:
> Unformatted:
----gnatsweb-attachment----
Content-Type: text/plain; name="foo.cc"
Content-Disposition: inline; filename="foo.cc"

#include <stdio.h>
int global_0 = 0;
int global_neg1 = -1;

int main()
{
    bool test = ((global_neg1&255)==global_neg1);
//    bool test = !(global_neg1&~255);
    if(test || global_0) {
	printf ("Guess again...\n");
    }
    return 0;
}


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