Re: Bug in Visual Studio 2005 compiler? - C / C++ / MFC Discussion Boards

Interesting bit about the size comments. I don't recall ever seeing that - maybe a different setting...

I trust that by now you realized that it is hard for us to reproduce the exact situation that led to the problem.

I still hold that it's dangerous to deduce anything from optimized code, short of print statements or similar right in the original code: you'll never know just what the compiler did, and why, to optimize your memory footprint and performance. It's difficult to draw correlations from assembler to the original code, and the expectations that come with it.

The code you posted doesn't look very complex. About the only optimization I would anticipate is that some of the local variables would be stored in register only, rather than on the stack. But then, optimizers work in mysterious ways - you'll never know what kind of optimization they can come up with until you see the code...

GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)

Interesting bit about the size comments. I don't recall ever seeing that - maybe a different setting...

Which compiler do you use? I only asked it to generate asm file with source code. All the rest was default settings (Sorry, later I added /Zi to generate debugger's symbols in release version. But it does not affect on generation of asm listing)

I trust that by now you realized that it is hard for us to reproduce the exact situation that led to the problem.

I do. You have no choice but to trust me (and stop looking where is light!).

I still hold that it's dangerous to deduce anything from optimized code, short of print statements or similar right in the original code: you'll never know just what the compiler did, and why, to optimize your memory footprint and performance. It's difficult to draw correlations from assembler to the original code, and the expectations that come with it.

But I do know exactly, what compiler does!!! What do you think asm listing exists for? Correlation is very simple and there are no problems with it. No matter, which sort of optimization is in use, resulting exe file must do the same things, and no matter, which optimization was used.

The code you posted doesn't look very complex. About the only optimization I would anticipate is that some of the local variables would be stored in register only, rather than on the stack. But then, optimizers work in mysterious ways - you'll never know what kind of optimization they can come up with until you see the code...

I saw. And without it I would never find, where the problem is.

GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)

Even Straustrup claims, that there are situation, when goto is useful. Assume you need get out of nested loops...
do {
do {
do {
if(something is wrong)
goto Skip_loops;
} while (cond A)
} while(cond B)
} while(cond C)
Skip_loops:;

a_matseevsky wrote:
Any idea how could it be?

Perhaps nonstandard compiler options or linker options.

And I am rather certain that max() is a macro.

Which means that the only possible source of the problem would be an optimization.
Excluding of course some pointer bug, which could cause almost any problem.

And I am also certain that you could write a trivially small program that would demonstrate this, which is basically what has already been asked. If you cannot in fact demonstrate the same problem with the trivial program then it would point to some other problem in your application.

There is no way to demonstrate problem, if compiler is unstable. problem disappeared, when I replaced max() and min(), but occurred again after some insignificant change in code (which was made far from the procedure, where problem occurred!). I used default compiler's options. Later, when I was looking for the source of the problem, I created database and set /Zi switch for the release version. After these actions I was able to find the very command, which destroyed my data. The next was trivial- I find offsets of local variables and noticed, that some of them was incorrectly placed (one partially overlaps another). This is the bug in compiler, not mine.

a_matseevsky wrote:
This is the bug in compiler, not mine.

Then go and tell Microsoft about it, as I already suggested, since you refuse to show us any of the code which causes the problem.

Veni, vidi, abiit domum

I will. I've just registered at M$'s site. And I got you all info to understand, where the problem is. For me situation is clear. What I got, running release under debugger:

lrct {top=30 bottom=958 left=162 right=750}

0041A88F fst qword ptr [ebp-0D4h]

lrct {top=30 bottom=1080213504 left=162 right=0}

Here you may see, what made one single command- RECT structure before and after this command. What additional info do you need? Prefix "tv" definitely means Temporarily Variables. This variable was placed too close to storage of RECT structure and fst overwrote it. If you cannot see it, I wash my hands. If you do not know, which bytes fst overwrite, sorry, man- there are problems, which obviously cannot be solved at the level of source code. One must dig deeper. And to do it, you must know many things, like assembler, structure of stack frame and so on and so on.
(lrct was stored at ebp-0DCh)

a_matseevsky wrote:
there are problems, which obviously cannot be solved at the level of source code

But without the source code it is impossible to understand the structure of the program or why these variables are being allocated the way they are. You keep saying you have given us all the information but all you have shown is two or three lines of assembler without any context, so there is no way we can begin to understand what is happening or why. I must confess I am at a total loss to understand why you refuse to provide the information that we have asked for so we can try and help you.

Veni, vidi, abiit domum

You cannot help me. How could you help me, if you even do not know, which bytes overwrites fst command? I do not send my procedure only because of its size. And I know now, where the problem is. Well, if you think that the problem is in my code, look at two small pieces and try to understand, why and which one works properly, but the other does not. I fixed the problem (well, not completely- I only rewrote my code, using my knowledge about compiler's incorrect behavior. lrct was removed at all.)

Variant A:
int xmin=xf+(fct*pdpr->m_croprect[Right].left-xf-ddi)*(xl-xf)/(yr1-yr0);
int xmax=xl+(fct*pdpr->m_croprect[Left].right-xl)*(xl-xf)/(yl1-yl0);
h=0.5*pdpr->m_croprect[Left].top+0.5*pdpr->m_croprect[Left].bottom;
pdpr->GetKorrHorz2D(xfirst, xlast, Dv, Dh, h, ddi, yh, yv, yhs, yvs, pKorr, ph, pv);
pdpr->GetCorrLineH2(xfirst, xlast, h, ddi, yh, yv, yh, yv, ph, pv, 0, 0);
yl0=xf+0.5*yh[xf];
yl1=xl+0.5*yh[xl];
yr0=xf+ddi-0.5*yh[xf];
yr1=xl+ddi-0.5*yh[xl];
int xl2=xf+(fct*pdpr->m_croprect[Right].left-xf-ddi)*(xl-xf)/(yr1-yr0);
if(xl2>xmin)
xmin=xl2;
int xr2=xl+(fct*pdpr->m_croprect[Left].right-xl)*(xl-xf)/(yl1-yl0);
if(xmax>xr2)
xmax=xr2;
h=0.1*pdpr->m_croprect[Left].top+0.9*pdpr->m_croprect[Left].bottom;
pdpr->GetKorrHorz2D(xfirst, xlast, Dv, Dh, h, ddi, yh, yv, yhs, yvs, pKorr, ph, pv);
pdpr->GetCorrLineH2(xfirst, xlast, h, ddi, yh, yv, yh, yv, ph, pv, 0, 0);
yl0=xf+0.5*yh[xf];
yl1=xl+0.5*yh[xl];
yr0=xf+ddi-0.5*yh[xf];
yr1=xl+ddi-0.5*yh[xl];
int xl3=xf+(fct*pdpr->m_croprect[Right].left-xf-ddi)*(xl-xf)/(yr1-yr0);
if(xl3>xmin)
xmin=xl3;
int xr3=xl+(fct*pdpr->m_croprect[Left].right-xl)*(xl-xf)/(yl1-yl0);
if(xmax>xr3)
xmax=xr3;
xfirst=xmin+0.02*(xmax-xmin);
xlast=xmax-0.02*(xmax-xmin);

Variant B:

int xl1=xf+(fct*pdpr->m_croprect[Right].left-xf-ddi)*(xl-xf)/(yr1-yr0);
int xr1=xl+(fct*pdpr->m_croprect[Left].right-xl)*(xl-xf)/(yl1-yl0);
h=0.5*pdpr->m_croprect[Left].top+0.5*pdpr->m_croprect[Left].bottom;
pdpr->GetKorrHorz2D(xfirst, xlast, Dv, Dh, h, ddi, yh, yv, yhs, yvs, pKorr, ph, pv);
pdpr->GetCorrLineH2(xfirst, xlast, h, ddi, yh, yv, yh, yv, ph, pv, 0, 0);
yl0=xf+0.5*yh[xf];
yl1=xl+0.5*yh[xl];
yr0=xf+ddi-0.5*yh[xf];
yr1=xl+ddi-0.5*yh[xl];
int xl2=xf+(fct*pdpr->m_croprect[Right].left-xf-ddi)*(xl-xf)/(yr1-yr0);
int xr2=xl+(fct*pdpr->m_croprect[Left].right-xl)*(xl-xf)/(yl1-yl0);
h=0.1*pdpr->m_croprect[Left].top+0.9*pdpr->m_croprect[Left].bottom;
pdpr->GetKorrHorz2D(xfirst, xlast, Dv, Dh, h, ddi, yh, yv, yhs, yvs, pKorr, ph, pv);
pdpr->GetCorrLineH2(xfirst, xlast, h, ddi, yh, yv, yh, yv, ph, pv, 0, 0);
yl0=xf+0.5*yh[xf];
yl1=xl+0.5*yh[xl];
yr0=xf+ddi-0.5*yh[xf];
yr1=xl+ddi-0.5*yh[xl];
int xl3=xf+(fct*pdpr->m_croprect[Right].left-xf-ddi)*(xl-xf)/(yr1-yr0);
int xr3=xl+(fct*pdpr->m_croprect[Left].right-xl)*(xl-xf)/(yl1-yl0);
int xmin=xl1;
int xmax=xr1;
if(xl2>xmin)
xmin=xl2;
if(xl3>xmin)
xmin=xl3;
if(xmax>xr2)
xmax=xr2;
if(xmax>xr3)
xmax=xr3;
xfirst=xmin+0.02*(xmax-xmin);
xlast=xmax-0.02*(xmax-xmin);

Thank you, all becomes clear.

Veni, vidi, abiit domum

Here you are. I could you recommend one useful book, written by John Robbins- "Debugging Applications". If you haven't read it, you'll find there many useful things.

Thanks but I have enough books on my reading list already.

Veni, vidi, abiit domum

It is only my suggestion. There are few books on debugging, and this one is really useful.

Richard, you have the patience of a saint.

We can’t stop here, this is bat country - Hunter S Thompson RIP

Please tell my wife and children. Smile | :)

Veni, vidi, abiit domum

a_matseevsky wrote:
There is no way to demonstrate problem, if compiler is unstable. problem disappeared

Compilers are not "unstable" in the way you are suggesting. If the environment is the same and stable and the compiler is same and the source (and build environment) is the same then excluding so runtime constants such as timestamps and hashes the output is the same.

a_matseevsky wrote:
when I replaced max() and min(), but occurred again after some insignificant change in code (which was made far from the procedure, where problem occurred!)....This is the bug in compiler, not mine.

Sorry then but that statement suggests that there is no other possible reason except that you have a pointer bug. Your application is misusing something somewhere.

The nature of max and what it does in terms of integers has not changed in years. One can't do much more in terms of optimizations with it, since for the most part it is the how the integers themselves are located and not the execution that can be optimized (and I only mention that because optimization was the only other explanation.)

However pointer bugs can impact almost anything in the application. And although a pointer bug might cause a problem where the actual pointer is in use it can in fact only show up far from the buggy code. (And I know this from experience not conjecture.)

It also doesn't need to reflect anything in the code that you changed in that it doesn't need to have a pointer in it. What matters is that you changed the execution path (by definition that is what code changes mean) and because of that something that could have been a bug but undetected for months or even years now causes some unexpected failure.

And I want to emphasize again that the pointer bug could be anywhere. The behavior you are seing is a symptom not a cause.

I can only recommend to you reread the whole discussion. I do know now, where the problem is. OK, compiler is absolutely stable. No problem with it. But it works incorrectly. It reserves some places in a stack for temporary variables. In fact, these variables stores content of co-processor's registers. Some of such temporary variables overlap (partially or completely) another local variables. It might be no problem- some local variables are visible only within some block, not within the whole procedure. If execution of code leaves some block (part of code within such {} brackets), all variables, declared within this block, becomes inaccessible and their place in a stack may be rewritten by another local variable. But compiler creates exe file, which performs this op even when some local variable is visible and accessible!!! And it happens not only with RECT structure, but with some of other local variables too. I saw this process, when I was running release version under debugger. Look up, where I placed piece of my code. Variable "h" was rewritten at least once. If it is not a compiler's bug, I'm definitely an elefant.

a_matseevsky wrote:
But it works incorrectly.

I suggest you reread my post - pointer bugs can have an impact FAR later in the code.

a_matseevsky wrote:
And it happens not only with RECT structure, but with some of other local variables too.

Don't know how to state this more clearly.

Either you have a pointer bug or there is a compiler problem. If the latter then reducing the code will demonstrate it AND changing code far from it and unrelated will NOT impact it.

Conversely if the former then you will not be able to reduce it because the code that you are looking at is not the source of the problem.

There was no pointer bugs at all. I declared no pointers, but the structure-
RECT lrct.
And some local variables too.
Compiler allocated them in a stack and added some temporary variables. In a such way:

_lrct$=-212;
tv5476=-204;

It is a time bomb, which might explode in any time. And it did. That's all. So simple.

a_matseevsky wrote:
I declared no pointers

You have a C++ application and do not use pointers ANYWHERE in the application?

(Again is has NOTHING to do with pointers directly associated with the code where you think the bug is.)

I do not think where the bug is. I know it. I know exactly where variables was incorrectly allocated in stack and which command overwrote data. What are you talking about pointers? I do use them, of course, but they was not the cause. It is pointless to discuss here what might happens, ignoring all available info.

a_matseevsky wrote:
What are you talking about pointers?

Either you didn't read what I said in my previous replies or didn't understand what I said.

jschell wrote:
Either you didn't read what I said in my previous replies or didn't understand what I said.

I did. It was you who did not read (or did not understand what he read). Look at quotes from your messages:

I want to emphasize again that the pointer bug could be anywhere. The behavior you are seing is a symptom not a cause.

Don't know how to state this more clearly.

Either you have a pointer bug or there is a compiler problem. If the latter then reducing the code will demonstrate it AND changing code far from it and unrelated will NOT impact it.

Conversely if the former then you will not be able to reduce it because the code that you are looking at is not the source of the problem.

You have a C++ application and do not use pointers ANYWHERE in the application?

(Again is has NOTHING to do with pointers directly associated with the code where you think the bug is.)

There are empty words about what might happens. Nothing common with real situation.
Compiler placed temporary variable too close to another one and when the tv5476 was used, RECT structure was partially overwritten. So simple. It is no pointer's problem- the compiler's one.

a_matseevsky wrote:
I did. It was you who did not read

I read the following which you posted.

"but occurred again after some insignificant change in code (which was made far from the procedure, where problem occurred!)"

Presumably you do not understand what that statement means in terms of what I said and in terms of what you are claiming.

jschell wrote:
I read the following which you posted.

"but occurred again after some insignificant change in code (which was made far from the procedure, where problem occurred!)"

Presumably you do not understand what that statement means in terms of what I said and in terms of what you are claiming.

I do know. Assume you have a pointer, which points yo some array. Sentence like *(p+k)=a may cause a problem if k is greater than array's size. This is the typical problem, associated with pointers. In my case all was different. What I wrote clearly meant that I had changed the source C++ code, then compiler built executable and the problem disappeared. Then I changed code again, recompiled it and the problem occurred again. That's just what I meant. Where do you see pointers? Moreover, at that very moment when I wrote aforementioned text, I did not know, where the problem was. I only noticed, that code worked correctly when I had excluded max and min. But problem occurred again! The most remarkable was the fact that I did not change suspicious procedure- changes had made far from it. But if one single line of code has been changed, compiler compiles the whole file with this line of code. I got compiler to build release with debug info and ran procedure under debugger. This was how I found where the problem is- I noticed that very command, which overwrote RECT structure and immediately found, why it happened. That's all.

a_matseevsky wrote:
Where do you see pointers?

Explaining it again...
- Pointer errors can show up far from the code where the bug actually is.
- Pointer errors do NOT automatically show up. An application can have a pointer error for years and run without problem.
- Changing code changes the execution path. BECAUSE of that a pointer error that previously did not impact the application can now impact the application.

Feel free to explain yourself, excluding pointer errors, why the code you change some where completely different is now causing this compiler bug to show up now.