Your bug reports play an essential role in making GCC reliable.
When you encounter a problem, the first thing to do is to see if it is already known. Refer to Chapter 11 Known Causes of Trouble with GCC. If it isn't known, then you should report the problem.
If you are not sure whether you have found a bug, here are some guidelines:
If the compiler gets a fatal signal, for any input whatever, that is a compiler bug. Reliable compilers never crash.
If the compiler produces invalid assembly code, for any input whatever (except an asm statement), that is a compiler bug, unless the compiler reports errors (not just warnings) which would ordinarily prevent the assembler from being run.
If the compiler produces valid assembly code that does not correctly execute the input source code, that is a compiler bug.
However, you must double-check to make sure, because you may have a program whose behavior is undefined, which happened by chance to give the desired results with another C or C++ compiler.
For example, in many nonoptimizing compilers, you can write x; at the end of a function instead of return x;, with the same results. But the value of the function is undefined if return is omitted; it is not a bug when GCC produces different results.
Problems often result from expressions with two increment operators, as in f (*p++, *p++). Your previous compiler might have interpreted that expression the way you intended; GCC might interpret it another way. Neither compiler is wrong. The bug is in your code.
After you have localized the error to a single source line, it should be easy to check for these things. If your program is correct and well defined, you have found a compiler bug.
If the compiler produces an error message for valid input, that is a compiler bug.
If the compiler does not produce an error message for invalid input, that is a compiler bug. However, you should note that your idea of "invalid input" might be my idea of "an extension" or "support for traditional practice".
If you are an experienced user of one of the languages GCC supports, your suggestions for improvement of GCC are welcome in any case.