MSVC++ 6.0 Complier optimization bug??

[jryden]

I wrote the following code into a console program in Visual C++6. If
you build a release build and select custom optimizations and select
'general optimizations' as the only optimization then you will end up
with incorrect results. Every line seems to be important, even the
unrelated bits. Any one have the time or inclination to try this out
for themselves and try to explain it?

#include "stdafx.h"
#include <string>

int main(int argc, char* argv[])
{
std::string s1; <- This is important

for(int i = 0; i<4; i++)
{
int shift = i*4; <- This is important
short temp = shift; <- This is important
printf("Generated %d\n", temp);
}

return 0;


while(1) <- This is important, take it out and the problem
disappears
{
}
}

The result of this program run with the specified optimizations is as
follows:
Generated 0
Generated 0
Generated 0
Generated 0

Note: This is (very) stripped down version of some production code I
had written that failed due to this odd bug. For now I plan to just
remove the optimizations until a proper fix can be determined. I hope
someone has some insight.

 

[jryden]

Of course I meant COMPILER....

complier postings should go into alt.tools...anyway, the posting is
serious

 

[Noah Roberts]

Of course I meant COMPILER....

complier postings should go into alt.tools...anyway, the posting is
serious

I can't recreate the issue in 7.1

 

[pillbug]

I compiled the following code using vc6sp5 and could not reproduce
the error.

cl -GX -Og test.cpp

#include <cstdio>
#include <string>

int main(int argc, char* argv[])
{
std::string s1; // <- This is important

for(int i = 0; i<4; i++)
{
int shift = i*4; // <- This is important
short temp = shift; // <- This is important
printf("Generated %d\n", temp);
}

return 0;

while(1) // <- This is important, take it out and the problem
disappears
{
}
}

 

[Victor Bazarov]

I wrote the following code into a console program in Visual C++6. If
you build a release build and select custom optimizations and select
'general optimizations' as the only optimization then you will end up
with incorrect results. Every line seems to be important, even the
unrelated bits. Any one have the time or inclination to try this out
for themselves and try to explain it?

The optimizer of VC6 is buggy. I've seen similare unpleasant things.

#include "stdafx.h"
#include <string>

int main(int argc, char* argv[])
{
std::string s1; <- This is important

for(int i = 0; i<4; i++)
{
int shift = i*4; <- This is important
short temp = shift; <- This is important
printf("Generated %d\n", temp);
}

return 0;


while(1) <- This is important, take it out and the problem
disappears
{
}
}

The result of this program run with the specified optimizations is as
follows:
Generated 0
Generated 0
Generated 0
Generated 0

Note: This is (very) stripped down version of some production code I
had written that failed due to this odd bug. For now I plan to just
remove the optimizations until a proper fix can be determined. I hope
someone has some insight.

Try declaing 'temp' or 'shift' (or both) "volatile". And keep the
optimization. See what happens.

V

 

[peter koch]

jryden skrev:

I wrote the following code into a console program in Visual C++6. If
you build a release build and select custom optimizations and select
'general optimizations' as the only optimization then you will end up
with incorrect results.

[snip]
This is off topic but anyway.
Do not optimise to heavily with VC++ 6.0. So far as I remember /Os or
/Ot was okay, but more elaborate optimisations went bad.

/Peter

 

[jryden]

No I tried that myself...it is specific to VC6

 

[jryden]

Ah...I'm not sure that I'm running the latest service pack...I make
sure and then retest the code.

Thanks.

 

[jryden]

Hum...doesn't change anything. Good to cover all the bases though. I
think I must have actually been running SP because the installation ran
too quick. Anyone know where you tell what SP is installed in 6.0.
The help|about doesn't show any version numbers...

 

[Victor Bazarov]

[...] Anyone know where you tell what SP is installed in
6.0. The help|about doesn't show any version numbers...

Please ask product-specific questions in the newsgroup dedicated
to your product. Example: microsoft.public.vc.ide_general.

V

 

[jryden]

Um...okay...sorry, it just followed the thread of suggestions. Please
disregard the question.

In that vein I tried your suggestion to use 'volatile'. This solves
the problem despite that fact that these are not actually volatile
variables. This is a better solution than my current solution which is
to simply turn off the optimizations.

Since I have a viable solution please consider this thread closed and
thanks for the help.

James

 

[Default User]

No I tried that myself...it is specific to VC6

Tried what? See below.



Brian