compiler: reduce calls to TypedTreeHasher

We need to check if our inference reached a fixed point. This is checked
using hash of the types in the AST, which is very slow. This patch
avoids computing the hash if we can make sure that the AST is definitely
changed, which is when we parse a new function.

For some simple programs with many functions, this can significantly
reduce the compile time by up to ~30%.

artiq/compiler/embedding.py

@@ -737,6 +737,7 @@ class Stitcher:
 
         self.embedding_map = EmbeddingMap()
         self.value_map = defaultdict(lambda: [])
+        self.definitely_changed = False
 
     def stitch_call(self, function, args, kwargs, callback=None):
         # We synthesize source code for the initial call so that
@@ -757,14 +758,20 @@ class Stitcher:
         old_attr_count = None
         while True:
             inferencer.visit(self.typedtree)
+            if self.definitely_changed:
+                changed = True
+                self.definitely_changed = False
+            else:
                 typedtree_hash = typedtree_hasher.visit(self.typedtree)
                 attr_count = self.embedding_map.attribute_count()
-
+                changed = old_attr_count != attr_count or \
-            if old_typedtree_hash == typedtree_hash and old_attr_count == attr_count:
+                          old_typedtree_hash != typedtree_hash
-                break
                 old_typedtree_hash = typedtree_hash
                 old_attr_count = attr_count
 
+            if not changed:
+                break
+
         # After we've discovered every referenced attribute, check if any kernel_invariant
         # specifications refers to ones we didn't encounter.
         for host_type in self.embedding_map.type_map:
@@ -891,6 +898,9 @@ class Stitcher:
             return types.TVar()
 
     def _quote_embedded_function(self, function, flags):
+        # we are now parsing new functions... definitely changed the type
+        self.definitely_changed = True
+
         if isinstance(function, SpecializedFunction):
             host_function = function.host_function
         else: