Given the relative simplicity of building C compiler (compared to C++), it's not surprising that the web is almost awash with different C compilers. I've mentioned Tiny C Compiler (TCC) which generates code for Windows and 64 bit CPUs. If you want to get immersed in the world of compilers (I did back before games), TCC is probably a very good place to start as it supports much of the C99 standard. It's also very fast at compiling: 859,000 lines per second on a 2.4 Ghz Pentium 4; that's 9 x faster than GCC! The penalty for fast compilation speed (made possible by code generation in a single pass) is of course lesser optimizations but for learning it's ideal.
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