Compilers

Virtual functions are one of C++’s core features, enabling runtime polymorphism. Most C++ programmers use them regularly, yet few understand how they work in practice. What does the compiler actually generate when you declare a function virtual? How does the program figure out which implementation to call at runtime? Where is the vtable data actually stored? This blog post is focused on answering these questions. The C++ standard specifies behavior but not implementation. This post describes the Itanium C++ ABI used by most platforms, with the notable exception of Microsoft MSVC. ...

Recently, I was bored and decided to test the performance of a 4x4 matrix multiply. I wanted to compare how GCC and Clang optimize the same code with different flags. So I manually wrote a benchmark harness using a neat trick: a C file that’s also a valid shell script. The idea is simple. You embed shell commands in C comments at the top of the file. When you run sh benchmark.c, the shell sees the commands and executes them to compile and run the benchmark. When the C compiler processes it, the commands are just comments. ...

I recently implemented BigObj COFF file parsing in cgo (golang/go#24341). In the process, I quickly discovered that Microsoft doesn’t document the binary format anywhere. Their official documentation is the only reference they have to BigObj as far as I can tell, and it doesn’t say anything about the binary format. I didn’t see any other blogs or resources covering this topic either. I figured it out by reading binutils and LLVM source code, so I’m documenting what I learned while the knowledge is still fresh in my memory. ...

Cross compilation is a common task during development, but different compilers and programming languages handle it in their own ways, and I wanted to write about the various flavors of trade-offs and design decisions that you will find across different tooling. I feel like I have absorbed a lot of information about how cross compilation works across different targets, tools and languages, so I figured it was time to condense my knowledge into a blog post. This is not a tutorial, but it still contains practically applicable knowledge. I don’t claim to get every detail correct, merely explaining how things work to my understanding. ...