Are GCC and Clang parsers really handwritten?

Clang's parser is a hand-written recursive-descent parser, as are several other open-source and commercial C and C++ front ends.

Clang uses a recursive-descent parser for several reasons:

• Performance: a hand-written parser allows us to write a fast parser, optimizing the hot paths as needed, and we're always in control of that performance. Having a fast parser has allowed Clang to be used in other development tools where "real" parsers are typically not used, e.g., syntax highlighting and code completion in an IDE.
• Diagnostics and error recovery: because you're in full control with a hand-written recursive-descent parser, it's easy to add special cases that detect common problems and provide great diagnostics and error recovery (e.g., see http://clang.llvm.org/features.html#expressivediags) With automatically generated parsers, you're limited to the capabilities of the generator.
• Simplicity: recursive-descent parsers are easy to write, understand, and debug. You don't need to be a parsing expert or learn a new tool to extend/improve the parser (which is especially important for an open-source project), yet you can still get great results.

Overall, for a C++ compiler, it just doesn't matter much: the parsing part of C++ is non-trivial, but it's still one of the easier parts, so it pays to keep it simple. Semantic analysis---particularly name lookup, initialization, overload resolution, and template instantiation---is orders of magnitude more complicated than parsing. If you want proof, go check out the distribution of code and commits in Clang's "Sema" component (for semantic analysis) vs. its "Parse" component (for parsing).