I'm writing a program in C++/Qt which contains a graph file parser. I use g++ to compile the project.
While developing, I am constantly comparing the performance of my low level parser layer between different compiler flags regarding optimization and debug information, plus Qt's debug flag (turning on/off qDebug() and Q_ASSERT()).
Now I'm facing a problem where the only correctly functioning build is the one without any optimization. All other versions, even with -O1, seem to work in another way. They crash due to unsatisfied assertions, which are satisfied when compiled without a -O... flag. The code doesn't produce any compiler warning, even with -Wall.
I am very sure that there is a bug in my program, which seems to be only harmful with optimization being enabled. The problem is: I can't find it even when debugging the program. The parser seems to read wrong data from the file. When I run some simple test cases, they run perfectly. When I run a bigger test case (a route calculation on a graph read directly from a file), there is an incorrect read in the file which I can't explain.
Where should I start tracking down the problem of this undefined behavior? Which optimization methods are possibly involved within this different behavior? (I could enable all flags one after the other, but I don't know that much compiler flags but -O... and I know that there are a lot of them, so this would need a very long time.) As soon as I know which type the bug is of, I am sure I find it sooner or later.
You can help me a lot if you can tell me which compiler optimization methods are possible candidates for such problems.
There are a few classes of bugs that commonly arise in optimized builds, that often don't arise in debug builds.
Un-initialized variables. The compiler can catch some but not all. Look at all your constructors, look at global variables. etc. Particularly look for uninitialized pointers. In a debug build memory is reset to zero, but in a release build it isn't.
Use of temporaries that have gone out of scope. For example when you return a reference to a local temporary in a function. These often work in debug builds because the stack is padded out more. The temporaries tend to survive on the stack a little longer.
array overruns writing of temporaries. For example if you create an array as a temporary in a function and then write one element beyond the end. Again, the stack will have extra space in debug ( for debugging information ) and your overrun won't hit program data.
There are optimizations you can disable from the optimized build to help make debugging the optimized version easier.
-g -O1 -fno-inline -fno-loop-optimize -fno-if-conversion -fno-if-conversion2 \
-fno-delayed-branch
This should make stepping through your code in the debugger a little easier to follow.
Another suggestion is that if the assertions you have do not give you enough information about what is causing the problem, you should consider adding more assertions. If you are afraid of performance issues, or assertion clutter, you can wrap them in a macro. This allows you to distinguish the debugging assertions from the ones you originally added, so they can be disabled or removed from your code later.
1) Use valgrind on the broken version. (For that matter, try valgrind on the working version, maybe you'll get lucky.)
2) Build the system with "-O1 -g" and step through your program with gdb. At the crash, what variable has an incorrect value? Re-run your program and note when that variable is written to (or when it isn't and should have been.)
来源：https://stackoverflow.com/questions/11423826/compiler-optimization-makes-program-crash