I posted this in the gcc help forums, but I think it belongs in this
one since its a g++ issue.
Hi,
I've encountered a problem that may be a compiler optimization bug
(with branch prediction). So far, I can reproduce it on Linux64 with g
+
+ 3.4.3:
nagara:/home/vmoskovi>g++ --version
g++ (GCC) 3.4.3 20050227 (Red Hat 3.4.3-22.1)
Copyright (C) 2004 Free Software Foundation, Inc.
Here is a simple program:
#define COPYLONG(pDstAddr, pSrcAddr) \
((int *) (pDstAddr))[0] = ((int *)(pSrcAddr))[0]; \
((int *) (pDstAddr))[1] = ((int *)(pSrcAddr))[1];
#include <iostream.h>
#include <stdio.h>
inline int test()
{
long long dataAddr = 5;
long long dupAddr = 0;
COPYLONG(&dupAddr, &dataAddr);
if (dupAddr == 0)
{
printf("IF\n");
}
else
{
printf("ELSE\n");
}
return 0;
}
main()
{
test();
return 0;
}
The expected output is ELSE since dupAddr should be set to 5. Here
is
the output:
nagara:/home/vmoskovi>g++ -O1 jt.cpp
In file included from /usr/lib/gcc/x86_64-redhat-linux/
3.4.3/../../../../include/c++/3.4.3/backward/iostream.h:31,
from jt.cpp:7:
/usr/lib/gcc/x86_64-redhat-linux/3.4.3/../../../../include/c++/3.4.3/
backward/backward_warning.h:32:2: warning: #warning This file
includes
at least one deprecated or antiquated header. Please consider using
one of the 32 headers found in section 17.4.1.2 of the C++ standard.
Examples include substituting the <X> header for the <X.h> header for
C
++ includes, or <iostream> instead of the deprecated header
<iostream.h>. To disable this warning use -Wno-deprecated.
nagara:/home/vmoskovi>a.out
ELSE
nagara:/home/vmoskovi>g++ -O2 jt.cpp
In file included from /usr/lib/gcc/x86_64-redhat-linux/
3.4.3/../../../../include/c++/3.4.3/backward/iostream.h:31,
from jt.cpp:7:
/usr/lib/gcc/x86_64-redhat-linux/3.4.3/../../../../include/c++/3.4.3/
backward/backward_warning.h:32:2: warning: #warning This file
includes
at least one deprecated or antiquated header. Please consider using
one of the 32 headers found in section 17.4.1.2 of the C++ standard.
Examples include substituting the <X> header for the <X.h> header for
C
++ includes, or <iostream> instead of the deprecated header
<iostream.h>. To disable this warning use -Wno-deprecated.
nagara:/home/vmoskovi>a.out
IF
Using optimization level 01, the output is correct. However, using
optimization level 02 it takes the wrong branch. I suspect that the
optimizer doesn't realize that COPYLONG modifies dupAddr. Is this a
known issue? Has anyone else encountered this problem?
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