Add some documentation about implemented extensions

2 files changed, 68 insertions, 3 deletions

diff --git a/README.md b/README.md
index 561a4e8..85ea24b 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,9 @@
 [![builds.sr.ht status](https://builds.sr.ht/~mcf/cc.svg)](https://builds.sr.ht/~mcf/cc)
 
-This is a C11 compiler using [QBE] as a backend. It is released under the
-[ISC] license.
+This is a [C11] compiler using [QBE] as a backend. It is released under
+the [ISC] license.
+
+Several GNU C [extensions] are also implemented.
 
 There is still much to do and some parts of the code are a little rough,
 but it currently implements most of the language, is self-hosting,
@@ -12,7 +14,7 @@ It was inspired by several other small C compilers including [8cc],
 
 ## Requirements
 
-The compiler itself is written in standard [C11] and can be built with
+The compiler itself is written in standard C11 and can be built with
 any conforming C11 compiler.
 
 The POSIX driver depends on POSIX.1-2008 interfaces, and the `Makefile`
@@ -71,6 +73,7 @@ Please report any issues to https://todo.sr.ht/~mcf/cc-issues.
 [QBE]: https://c9x.me/compile/
 [C11]: http://port70.net/~nsz/c/c11/n1570.html
 [ISC]: https://git.sr.ht/~mcf/cc/blob/master/LICENSE
+[extensions]: https://git.sr.ht/~mcf/cc/tree/master/doc/extensions.md
 [8cc]: https://github.com/rui314/8cc
 [c]: https://github.com/andrewchambers/c
 [scc]: http://www.simple-cc.org/
diff --git a/doc/extensions.md b/doc/extensions.md
new file mode 100644
index 0000000..14ec71b
--- /dev/null
+++ b/doc/extensions.md
@@ -0,0 +1,62 @@
+# Extensions
+
+In addition to C11, several [GNU extensions] are implemented and more
+may be implemented in the future.
+
+## Implemented
+
+### [`__typeof__`]
+
+`__typeof__(E)` is a type specifier for the type of expression `E`. Arrays
+and function designator expressions do not decay into pointers, just
+like when used with `sizeof`.
+
+### Built-in functions and types
+
+- **`__builtin_alloca`**: Allocate memory on the stack.
+- **`__builtin_constant_p`**: Test whether the argument is a constant expression.
+- **`__builtin_inff`**: `float` positive infinity value.
+- **`__builtin_nanf`**: `float` quiet NaN value.
+- **[`__builtin_offsetof`]**: Return the offset of a member in a struct or union.
+- **`__builtin_types_compatible_p`**: Test whether the two types are compatible.
+- **`__builtin_va_arg`**: Built-in suitable for implementing the `va_arg` macro.
+- **`__builtin_va_copy`**: Built-in suitible for implementing the `va_copy` macro.
+- **`__builtin_va_end`**: Built-in suitible for implementing the `va_end` macro.
+- **`__builtin_va_list`**: Built-in suitable for implementing the `va_list` type.
+- **`__builtin_va_start`**: Built-in suitable for implementing the `va_start` macro.
+
+## Missing
+
+### [Statement expressions]
+
+In GNU C, you may use a compound statement as expressions when they are
+enclosed in parentheses. The last statement in the compound statement
+must be an expression statement, whose value is used as the result of
+the statement expression.
+
+### Empty declarations
+
+GNU C allows empty top-level declarations (i.e. `;`).
+
+### Empty initializer list
+
+GNU C allows empty initializer lists when initializing an object,
+for example
+
+```c
+struct {
+	int a, b;
+} s = {};
+```
+
+This behaves exactly the same as `{0}`.
+
+### Missing operand in conditional expression
+
+GNU C allows you to omit the second operand in conditional expressions,
+in which case the first operand is used. So `E1 ? : E2` behaves the same
+as `E1 ? E1 : E2`, except that `E1` is evaluated only once.
+
+[GNU extensions]: https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html
+[`__typeof__`]: https://gcc.gnu.org/onlinedocs/gcc/Typeof.html
+[Statement expressions]: https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html