Add a special #ifdef for decomp.me convenience

README.md

@@ -271,6 +271,20 @@ python3 ./m2c.py --visualize=c --context ctx.c -f my_fn my_asm.s > my_fn_c.svg
 python3 ./m2c.py --visualize=asm --context ctx.c -f my_fn my_asm.s > my_fn_asm.svg
 ```
 
+### Preprocessing
+
+There currently is a pseudo-macro in lieu of a full preprocessor that allows for the conditional switching of code in a context file. This allows for both m2c and e.g. a compiler to use the same context file if both need to define e.g. structs or typedefs slightly differently.
+
+```c
+#ifdef M2C
+...
+#else
+...
+#endif
+```
+
+Any other macros besides `#ifdef M2C` currently will fail, and you also need the `#else` between `#ifdef M2C` and `#endif` for the pattern to match.
+
 ### Migrating from `mips_to_c.py`
 
 This tool was originally known as `mips_to_c`. As part of the rename, deprecated command line arguments were removed.

m2c/c_types.py

@@ -647,6 +647,18 @@ def replacer(match: Match[str]) -> str:
     return re.sub(pattern, replacer, text)
 
 
+def process_ifdef(text: str) -> str:
+    pattern = re.compile(
+        r"^[ \t]*#ifdef[ \t]+M2C\s+"
+        r"(?P<ifdef_body>.*?)"
+        r"^[ \t]*#else\s+"
+        r".*?"
+        r"^[ \t]*#endif.*?$",
+        flags=re.DOTALL | re.MULTILINE,
+    )
+    return re.sub(pattern, lambda m: m.group("ifdef_body"), text)
+
+
 def strip_macro_defs(text: str) -> str:
     """Strip macro definitions from C source. m2c does not run the preprocessor,
     for a bunch of reasons:
@@ -740,6 +752,7 @@ def _build_typemap(source_paths: Tuple[Path, ...], use_cache: bool) -> TypeMap:
 
         source = add_builtin_typedefs(source)
         source = strip_comments(source)
+        source = process_ifdef(source)
         source = strip_macro_defs(source)
 
         ast, result_scope = parse_c(source, typemap.cparser_scope)