[Bug c++/38297] New: O2 causes invalid code

[andrew at warnux dot com]

I am compiling with g++ on a 64 bit Ubuntu OS.  I spent days trying to find out
why my programs kept crashing.  I found that when using O2 optimizations the
problem is there, when using O1 it is not a problem.

I am not using any compiler options other than stripping symbols, hiding
warnings, and O2.

Here is a very simple test case that causes the bug every time.

void RefTest(byte*& p)
{
++p;
}
void RefTest(char*& p)
{
++p;
}

char* c= "0123";
char* p= c;
RefTest((byte*&)p);
cout << p << endl; // 0123
RefTest(p);
cout << p << endl; // 123

return 0;


-- 
   Summary: O2 causes invalid code
   Product: gcc
   Version: 4.3.2
Status: UNCONFIRMED
  Severity: critical
  Priority: P3
 Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: andrew at warnux dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38297

[Bug c++/38297] O2 causes invalid code

[andrew at warnux dot com]

--- Comment #1 from andrew at warnux dot com  2008-11-28 00:32 ---
I guess you want this too:

gcc -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.3.2-1ubuntu11'
--with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.3
--program-suffix=-4.3 --enable-clocale=gnu --enable-libstdcxx-debug
--enable-objc-gc --enable-mpfr --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.3.2 (Ubuntu 4.3.2-1ubuntu11)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38297

[Bug c++/38297] O2 causes invalid code

[andrew at warnux dot com]

--- Comment #3 from andrew at warnux dot com  2008-11-28 01:59 ---
unsigned char

char and byte are the same size (1 byte), so why is there a problem?


-- 

andrew at warnux dot com changed:

   What|Removed |Added

 Status|WAITING |UNCONFIRMED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38297

[Bug c++/38297] O2 causes invalid code

[andrew at warnux dot com]

--- Comment #5 from andrew at warnux dot com  2008-11-28 02:33 ---
Thanks!  I guess I learned something new today.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38297

[Bug c++/38297] O2 causes invalid code

[andrew at warnux dot com]

--- Comment #6 from andrew at warnux dot com  2008-11-28 16:38 ---
I do have a couple concerns:

1. This one is MAJOR.  Without using -Wstrict-aliasing, I never see warnings
about this change.  I can't even begin to explain how bad that is.  The gcc
programmers made a big change (that is default compile) and there isn't even a
warning by default!  I wonder how many other things like this have been done.

2. Will other compilers compile this kind of code correctly?
inline bool IsNegative(float f)
{
union
{
float f;
int i;
} u;
u.f = f;
return u.i & 0x8000;
}
http://xania.org/200712/cpp-strict-aliasing


-- 

andrew at warnux dot com changed:

   What|Removed |Added

 Status|RESOLVED|UNCONFIRMED
 Resolution|DUPLICATE   |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38297

[Bug c++/38297] O2 causes invalid code

[andrew at warnux dot com]

--- Comment #9 from andrew at warnux dot com  2008-11-28 22:01 ---
I have another question.  I want to be able to detect if fno-strict-aliasing
was used when compiling.  Preferably at compile time but run time will be fine.
 How can I do this?


-- 

andrew at warnux dot com changed:

   What|Removed |Added

 Status|RESOLVED|UNCONFIRMED
 Resolution|DUPLICATE   |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38297

[Bug c++/38297] O2 causes invalid code

[andrew at warnux dot com]

--- Comment #10 from andrew at warnux dot com  2008-11-28 22:02 ---
The shortest answer possible will be fine.  I don't want to be an annoyance.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38297