|
Jolly good for you. You obviously won't need any help from us in the future then.
Veni, vidi, abiit domum
|
|
|
|
|
I need help. Question is, if any of you can give me some more or less valuable tip. When I asked my question the first time, I was not sure, that a bug really presents in Visual Studio. Now I know it. Problem is, what I have to do in such sorrowful situation. If someone else met such problem and fixed it, such person could share his experience. May be, this bug presents only in my own exemplar of Visual Studio- may be, it was damaged in some way and produces code with defects- in such case re-installation will solve this problem. Or such bug presents in each exemplar of Visual Studio 2005- in such case, I have to throw it away and buy 2008 or 2010. I know about a bug in Borland's compiler.
|
|
|
|
|
a_matseevsky wrote: I need help.
Then you could start by showing the actual code that is going wrong, as raised in your original question. Also you need to show the exact values of the variables that cause the problems, and the actual and expected result values.
Veni, vidi, abiit domum
|
|
|
|
|
I did it. Problem was caused by incorrect allocation of local variables in stack frame. This is what I hardly can change- it is a bug in compiler. The only important fact is that one value was replaced by another. Exact values of variables here mean nothing.
|
|
|
|
|
If you continue to refuse to provide the information we have asked for, then there is no way we can offer any suggestions as to what may be wrong. If you are convinced that this is a compiler bug then you should collect all the information and send it to Microsoft.
Veni, vidi, abiit domum
|
|
|
|
|
I clearly demonstrated, what cased problem in particular. I can repeat it again.
There was two local variables:
_lrct$ = -212 ; size = 16
tv5476 = -204 ; size = 8
The next command rewrites 8 bytes of _lrct$
fst QWORD PTR tv5476[ebp]
That's all info. Which data in particular was replaced and by what- means nothing. The only important fact is that data was replaced with something else.
|
|
|
|
|
You have not clearly demonstrated anything. Unless we see the source code there is nothing we can suggest. And, as I said before, if you are convinced that this is a compiler bug, then you should be sending it to Microsoft.
Veni, vidi, abiit domum
|
|
|
|
|
This is not the C/C++ source code.
There is a reason Richard is asking for it: it is all too common among people working "close to the metal" to "over-optimize" their code in a way that is simply wrong, or leads the compiler to produce inieffective or incorrect code. I'm not saying you did that, but you wouldn't be the first nor would you be the last.
Anyway, we can't decide where the error or bug is without seeing the actual C/C++ source code. Nor can anyone provide additional suggestions or help if you don't at the very least offer us the same information that your compiler got.
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)
|
|
|
|
|
There are a whole lot of questions that remain unanswered. Either may deliver the true reason for the bug you're experiencing. Compiler bugs are certainly possible, but rather unlikely.
You should really look at the problem again and consider some or all of the following questions:
1. Where did you get the information that
- _lrct$ refers to the start address of the RECT struct you're referring to
- the RECT struct is really 16 bytes in size
- the offsets you show are in fact relative to the same base address
2. Did you derive from your observation of these addresses that part of your data is overwritten, or did you check the actual RECT structure to verify that?
3. What are the original declarations of the C/C++ symbols corresponding to the two addresses _lrct$ and tv5476?
4. How were the two objects allocated?
5. Are you sure that one of them (the RECT) hasn't been deallocated in the meantime? Note that optimizers may discard variables before the end of their lifetime as seen in code if they realize it is no longer used!
I'm sure I could think of more questions, but this could be much more productive if we could see the actual 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)
|
|
|
|
|
|
Umm ... yes ... I kind of see it now
I think I'll go ... meditate or something
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)
|
|
|
|
|
Dear Stefan, it seems to me, that I have to remind you some theory. Local variables within some procedure may be addressed via ebp or esp. In my particular case they was addressed via ebp. It is much simpler to read such code, because ebp remains constant within a procedure. I some local variable is defined within some block, it becomes inaccessible, when eip leaves this particular block. Its place may be overwritten by something else, that's correct. But simultaneously debugger refuses to show this variable!!! But it showed it- therefore, lrct (16 byte RECT structure) was in its own block, was visible and accessible. But it was partially overwritten- 8 bytes was used as temporary storage for ST(0). That behavior cannot be taken as a normal one. And not lrct only- same fate waited h and maybe something else.
|
|
|
|
|
a_matseevsky wrote: within a procedure
This is just one of many bits of information you haven't really provided. Therefore the wide array of questions. I am well aware that many probably don't make a lot of sense to ask in your particular case if you see the exact code that the compiler used to produce the assembly.
But as repeatedly mentioned, you haven't provided that code, so all we can do is guess and poke in the dark.
In the meantime I've spotted the code that you posted in a different branch of this thread. It isn't the original code though, and doesn't provide sufficient insight to answer any of the questions I posted. You state that variant A doesn't work, but it doesn't use the max() function, so we have to assume that the assembler code generated from that doesn't match the bits you later found to be incorrect (if they are indeed that - we still do not know)
In the code you posted you use some variables that are only referenced over a couple of lines. They may not in fact be stored in the stack at all! As a result, the debugger will not show their contents in the optimized release code. Similarly, if you define a struct with data that is never referenced, the compiler may decide to optimize away the unneeded bits, reducing the size of the struct. You haven't posted the definition of the struct, nor where it's accessed, so it's impossible to tell if that is the case.
I could go on and easily bring up half a dozen or more other optimization techniques that you appear to be unaware of, and that will confound your ability to read useful information from the debugger alone within optimized code. But it's pointless.
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)
|
|
|
|
|
Stefan_Lang wrote:
In the meantime I've spotted the code that you posted in a different branch of this thread. It isn't the original code though, and doesn't provide sufficient insight to answer any of the questions I posted. You state that variant A doesn't work, but it doesn't use the max() function, so we have to assume that the assembler code generated from that doesn't match the bits you later found to be incorrect (if they are indeed that - we still do not know)
What do you mean, writing "it isn't the original code?" That I copied it from somewhere or what? I never stated, that variant A does not work. It is your idea. I asked, which works and which not (and why). You was not able to answer this question and preferred to ignore it.
In the code you posted you use some variables that are only referenced over a couple of lines. They may not in fact be stored in the stack at all! As a result, the debugger will not show their contents in the optimized release code. Similarly, if you define a struct with data that is never referenced, the compiler may decide to optimize away the unneeded bits, reducing the size of the struct. You haven't posted the definition of the struct, nor where it's accessed, so it's impossible to tell if that is the case.
All fields of the RECT structute was later used. If compiler decided that they are no more in use, it is just its bug. RECT structure is so well-known, that i did not placed its definition here. It contents 4 fields of type long- top, left, right and bottom. Just 16 bytes. I ran release version under debugger and I saw all assembler commands and order of their execution. Therefore, I know which variables was not stored in a stack and which temporary variables (which I did not declared!) was stored and where.
I could go on and easily bring up half a dozen or more other optimization techniques that you appear to be unaware of, and that will confound your ability to read useful information from the debugger alone within optimized code. But it's pointless.
Pointless is to repeat things like "there are more things on the Earth, on the heaven, than any dreamt of in our philosophy". What makes you think that I do not know about optimization? Stop walking around and answer to one simple question: why local variable, visible and accessible, was overwritten? You cannot name at least one more or less credible reason. If it is not visible, I couldn't get access to it and debugger wouldn't show it. Just this situation happens with variables, which are used in one-two lines of code. Debugger doesn't show them at all (worry not, I know such things).
|
|
|
|
|
You should be aware that asking us which optimized code would work and which wouldn't, without posting the full code with declarations and optimizer settings is ludicrous.
Therefore I decided to derive it based on you statements elsewhere: Variant B contains code that works by your own statement elsewhere. neither contains the max() funtion you refer to in your original question. So B is the working and A the not working - albeit not the original - version. Q. E. D.
As for what is pointless or not: advice is only pointless if it isn't heeded. You believe that the debugger shows you every information correctly? It doesn't. It can't. It's physically impossible.
I've dealt with optimizers 30 years ago: back then it wasn't to hard to anticipate what it would do, and in C you could often emulate pretty much the same without having to invoke the optimizer at all. meaning well optimized C code ran almost as well in debug mode as it did in release.
I've dealt with optimizers 20 years ago, and it got more tricky. Still, well optimized C-code often turned out to be near optimal.
I've dealt with optimizers 10 years ago, and the experience was very different: for one, the same optimizations in C code sometimes led to slower code, because it prevented the optimizer from performing extremely sophisticated optimizations that you'd never have thought of. Plus the size of the codebase made it impossible to optimize all of your code in that way anyway.
At that point I stopped trying to optimize my C/C++ code by hand.
Nowadays, when I check release code that doesn't do the same as debug code, I often find that half the variables aren't on stack, and some others get overwritten at unexpected moments. At least when I look at them through the debugger. It's the optimizer at work! What I usually do is pinpoint the location where everything goes south, then insert some print statements to ensure everything is as I expect it to be - and in 9 cases out of 10, something will not be as I expect it to be! And the debugger won't be able to show it!
That's why I keep telling you not to trust the debugger - and looking at the assembler code doesn't change that you're using the debugger.
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)
|
|
|
|
|
Stefan_Lang wrote:
Therefore I decided to derive it based on you statements elsewhere: Variant B contains code that works by your own statement elsewhere. neither contains the max() funtion you refer to in your original question. So B is the working and A the not working - albeit not the original - version. Q. E. D.
Again- I never claimed, which variant works and which does not. It was my question. And what "not the original version" means? I made my own investigation since the time, when I asked my first question. I changed code many times. I found, that no min nor max are not the source of the problem. There is no need in demonstrating of the rest of original code- it is the same for A and B. The difference is only here, in these few lines. And my question was- how could it be- both variants are correct. So you cannot answer to my question. Should I send you the answer or you will think for a while?
As for what is pointless or not: advice is only pointless if it isn't heeded. You believe that the debugger shows you every information correctly? It doesn't. It can't. It's physically impossible.
Why? Which part is hidden? Pipelines? Debugger demonstrates me all what I need to understand the core of situation. You must work really hard, if you want to cheat (or to trick- whichever word do you prefer) debugger. It happens mostly with self-modified code.
I've dealt with optimizers 30 years ago: back then it wasn't to hard to anticipate what it would do, and in C you could often emulate pretty much the same without having to invoke the optimizer at all. meaning well optimized C code ran almost as well in debug mode as it did in release.
I've dealt with optimizers 20 years ago, and it got more tricky. Still, well optimized C-code often turned out to be near optimal.
I've dealt with optimizers 10 years ago, and the experience was very different: for one, the same optimizations in C code sometimes led to slower code, because it prevented the optimizer from performing extremely sophisticated optimizations that you'd never have thought of. Plus the size of the codebase made it impossible to optimize all of your code in that way anyway.
At that point I stopped trying to optimize my C/C++ code by hand.
More than strange idea. Hi-level optimization cannot be performed by compiler. Low-level usually yes, but not always.
Nowadays, when I check release code that doesn't do the same as debug code, I often find that half the variables aren't on stack, and some others get overwritten at unexpected moments. At least when I look at them through the debugger. It's the optimizer at work! What I usually do is pinpoint the location where everything goes south, then insert some print statements to ensure everything is as I expect it to be - and in 9 cases out of 10, something will not be as I expect it to be! And the debugger won't be able to show it!
I should ask you, do you really know, how one can get debugger demonstrate all of what he needs? Do you use register's window? Co-processor's one? Debugger may demonstrate you memory at any address what you need. And when I run release under debugger, it demonstrates me result of disassembling of binary executable file- just what it is executing, BUT NOT MY SOURCE CODE! This is why I see just what is executed. And I can compare it with the listing, which contains source C++ and assembler. I can create release with debugger info and create separated file with database. Debugger (even part of Visual Studio) is very powerful one and been properly used, can give you all what you need and more that that. Sorry, but I printed variables too- many years ago. it was only because I did know how to work with debugger. Later I found that debugger offers me info which I needed in the more convenient way.
That's why I keep telling you not to trust the debugger - and looking at the assembler code doesn't change that you're using the debugger.
You may trust debugger, if you know, how to use it.
|
|
|
|
|
Well, I've just answered, but I do not see it... Well, let's go point by point
Stefan_Lang wrote:
You should really look at the problem again and consider some or all of the following questions:
1. Where did you get the information that
- _lrct$ refers to the start address of the RECT struct you're referring to
What do you think .asm files exist for? Compiler made them and I was able to see, where local and temporary variables was allocated.
- the RECT struct is really 16 bytes in size
Nice question, it comes to my mind too- but the same compiler was so kind, that showed me the size of a RECT structure
- the offsets you show are in fact relative to the same base address
You should know, that ebp register remains constant within any procedure! And in this particular case all local and temporary variables was addressed via ebp.
2. Did you derive from your observation of these addresses that part of your data is overwritten, or did you check the actual RECT structure to verify that?
I saw it, running my prog under debugger and I know which command (fst) overwrote aforementioned RECT (in fact, only right and bottom).
3. What are the original declarations of the C/C++ symbols corresponding to the two addresses _lrct$ and tv5476?
RECT lrct; the second variable is local storage, using to temporary storage of ST(0)
4. How were the two objects allocated?
I wrote it, but I can repeat it again. _lrct=-212 and tv5476=-204 (number a decimal ones!) It means, that command fst tv5476[ebp] overwrites 8 bytes- one half of lrct structure.
5. Are you sure that one of them (the RECT) hasn't been deallocated in the meantime? Note that optimizers may discard variables before the end of their lifetime as seen in code if they realize it is no longer used!
It would be possible, but in such case debugger refuses to show such variable and gives message like "expression cannot be evaluated". Moreover, compiler wouldn't let me to use a variable outside of the block, where it was declared.
I'm sure I could think of more questions, but this could be much more productive if we could see the actual code ...
You saw it. Answer a question, originally addressed to Richard- which variant (and why) refuses to work properly.
|
|
|
|
|
1.a) I realize that one is from the generated assembler code , but "I am able to see" is not an answer to my question. But never mind, you later wrote the original symbol is named _lrct, which I consider sufficient information at this point.
Considering the fact that symbols starting with '_' are common within windows system libaries and the MFC, and 'rct' is a common name fragment used for windows rectangle types and variables, I presume this is the Windows RECT struct[^] that you're talking about?
1b) Are you saying you used the sizeof() function? That would be the only way I know of the compiler can 'tell' you as much. Or if you derived it from bits of the assembler code, I have to trust your word for it - I don't don't know how to extract that kind of information.
1c) it could have been different procedures, or even different threads. Before your more recent answers there was no way to tell. So this isn ot (part of) the cause of the problem.
2 As mentioned in another post: you can't trust debugger output in optimized code! Some variables may not be stored in the stack at all, others may be overwritten before they expire, and addresses used to view a particular element may not contain the correct value, or the most recent state of the variable, due to caching, or memory optimization. The only way to be sure of the actual, current state of a variable, is print it out to the console or maybe log file.
4 I meant memory allocation, as in whether they are on the stack, the heap, or temporaries. But in light of the other responses this is no longer important.
5: see 2. - Don't trust the debugger in optimized code.
I still have some doubts the compiler has a bug - the symptom you describe is just too obvious. My guess is that it's a result of optimization. But with your responses you excluded a number of possible alternate causes. If you still feel it is a genuine bug you could report it to MS. However, I'm not sure whether they'll look into an 8 year old compiler when they have various newer versions to offer: if they find a bug, they'll probably only fix it in the newest version(s).
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)
|
|
|
|
|
Stefan_Lang wrote:
1.a) I realize that one is from the generated assembler code , but "I am able to see" is not an answer to my question. But never mind, you later wrote the original symbol is named _lrct, which I consider sufficient information at this point.
Not exactly. In my code was a simple line like
RECT lrct; compiler converted it into
tv6649 = -332 ; size = 4
_lrct$ = -328 ; size = 16
_sf$204092 = -312 ; size = 8
There is no "sizeof"; yeah this operator made me problems once or twice. Look at how here lrct is placed- just correctly (it is another procedure! I rewrote that one, which caused problems, but as I wrote, there was difference not 16 bytes, but only 8. (I cannot show you the code, which caused problem- I rewrote it. Wrong allocation can be found now only here, in this tread)
Considering the fact that symbols starting with '_' are common within windows system libaries and the MFC, and 'rct' is a common name fragment used for windows rectangle types and variables, I presume this is the Windows RECT struct[^] that you're talking about?
Positive. But _ (and $ as well) was added by compiler.
1b) Are you saying you used the sizeof() function? That would be the only way I know of the compiler can 'tell' you as much. Or if you derived it from bits of the assembler code, I have to trust your word for it - I don't don't know how to extract that kind of information.
I did not tell it- look up. Compiler wrote its size as a commentary in asm listing.
1c) it could have been different procedures, or even different threads. Before your more recent answers there was no way to tell. So this isn ot (part of) the cause of the problem.
2 As mentioned in another post: you can't trust debugger output in optimized code! Some variables may not be stored in the stack at all, others may be overwritten before they expire, and addresses used to view a particular element may not contain the correct value, or the most recent state of the variable, due to caching, or memory optimization. The only way to be sure of the actual, current state of a variable, is print it out to the console or maybe log file.
What makes you think so? I watch at assembler code (well plus the source one), when I run release under debugger. And it demonstrates me just what it does.
4 I meant memory allocation, as in whether they are on the stack, the heap, or temporaries. But in light of the other responses this is no longer important.
5: see 2. - Don't trust the debugger in optimized code.
I do. But as I wrote, I use assembler in such case. Without it I couldn't to find, where the problem is.
If one try to debug release, using source code only, he will be really surprised.
I still have some doubts the compiler has a bug - the symptom you describe is just too obvious. My guess is that it's a result of optimization. But with your responses you excluded a number of possible alternate causes. If you still feel it is a genuine bug you could report it to MS. However, I'm not sure whether they'll look into an 8 year old compiler when they have various newer versions to offer: if they find a bug, they'll probably only fix it in the newest version(s).
I'm sure that they do not. And I sure, that it is a bug- what else? Optimization, yeah, but it could be buggy too. Question however still remains, why this but affects only in one particular procedure. I saw a bug in Borland's compiler. This one was quite cute- I wrote some assembler code, but it removed one command from there.
|
|
|
|
|
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)
|
|
|
|
|
General 
News 
Suggestion 
Question 
Bug 
Answer 
Joke 
Praise 
Rant 
Admin 
Submit an article or tip
