9cc C compiler

Note: 9cc is no longer an active project, and the successor is
chibicc.

9cc is a successor of my 8cc C compiler.
In this new project, I'm trying to write code that can be understood
extremely easily while creating a compiler that generates reasonably
efficient assembly.

9cc has more stages than 8cc. Here is an overview of the internals:

1. Compiles an input string to abstract syntax trees.
2. Runs a semantic analyzer on the trees to add a type to each tree node.
3. Converts the trees to intermediate code (IR), which in some degree
   resembles x86-64 instructions but has an infinite number of registers.
4. Maps an infinite number of registers to a finite number of registers.
5. Generates x86-64 instructions from the IR.

There are a few important design choices that I made to keep the code as
simple as I can get:

• Like 8cc, no memory management is the memory management policy in 9cc.
  We allocate memory using malloc() but never call free().
  I know that people find the policy odd, but this is actually a
  reasonable design choice for short-lived programs such as compilers.
  This policy greatly simplifies code and also eliminates use-after-free
  bugs entirely.

• 9cc's parser is a hand-written recursive descendent parser, so that the
  compiler doesn't have any blackbox such as lex/yacc.

• I stick with plain old tools such as Make or shell script so that you
  don't need to learn about new stuff other than the compiler source code
  itself.

• We use brute force if it makes code simpler. We don't try too hard to
  implement sophisticated data structures to make the compiler run faster.
  If the performance becomes a problem, we can fix it at that moment.

• Entire contents are loaded into memory at once if it makes code simpler.
  We don't use character IO to read from an input file; instead, we read
  an entire file to a char array in a batch. Likewise, we tokenize a
  whole file in a batch rather than doing it concurrently with the parser.

Overall, 9cc is still in its very early stage. I hope to continue
improving it to the point where 9cc can compile real-world C programs such
as Linux kernel. That is an ambitious goal, but I believe it's achievable,
so stay tuned!