Switch scanner CPPDEFINES replacement algorithm

Now using the "modified" approach from the PR discussion: unroll the
dict comprehension and as we process replacements keep track if changes
were made, rather than doing the relatively more expensive dict-vs-dict
comparison at the end of each loop.

Signed-off-by: Mats Wichmann <[email protected]>

CHANGES.txt

@@ -167,9 +167,6 @@ RELEASE  VERSION/DATE TO BE FILLED IN LATER
       their values taken from the default in the variable description
       (if a variable was set to the same value as the default in one
       of the input sources, it is not included in this list).
-    - The C scanner now does (limited) macro replacement on the values in
-      CPPDEFINES, to improve results of conditional source file inclusion
-      (issue #4523).
     - The (optional) C Conditional Scanner now does limited macro
       replacement on the contents of CPPDEFINES, to improve finding deps
       that are conditionally included.  Previously replacement was only

SCons/Scanner/C.py

@@ -114,8 +114,19 @@ def _replace(mapping: Dict) -> Dict:
         old_ns = mapping
         loops = 0
         while loops < 5:  # don't recurse forever in case there's circular data
-            ns = {k: old_ns[v] if v in old_ns else v for k, v in old_ns.items()}
-            if old_ns == ns:
+            # this was originally written as a dict comprehension, but unrolling
+            # lets us add a finer-grained check for whether another loop is
+            # needed, rather than comparing two dicts to see if one changed.
+            again = False
+            ns = {}
+            for k, v in old_ns.items():
+                if v in old_ns:
+                    ns[k] = old_ns[v]
+                    if not again and ns[k] != v:
+                        again = True
+                else:
+                    ns[k] = v
+            if not again:
                 break
             old_ns = ns
             loops += 1