arglist: optimize flush_pre_post

pre_flush_set and post_flush_set are almost always empty, so we can use
extend() instead of a for...in loop to add the previous elements of
self._container.

We can also skip the conversion from deque to list since pre_flush is
always appended on the right side.

On a QEMU build the time spent in flush_pre_post goes from 1.4 to 0.5
seconds.

mesonbuild/arglist.py

@@ -119,7 +119,7 @@ class CompilerArgs(collections.abc.MutableSequence):
     # This correctly deduplicates the entries after _can_dedup definition
     # Note: This function is designed to work without delete operations, as deletions are worsening the performance a lot.
     def flush_pre_post(self) -> None:
-        pre_flush = collections.deque()   # type: T.Deque[str]
+        new = list()                      # type: T.List[str]
         pre_flush_set = set()             # type: T.Set[str]
         post_flush = collections.deque()  # type: T.Deque[str]
         post_flush_set = set()            # type: T.Set[str]
@@ -128,7 +128,7 @@ class CompilerArgs(collections.abc.MutableSequence):
         for a in self.pre:
             dedup = self._can_dedup(a)
             if a not in pre_flush_set:
-                pre_flush.append(a)
+                new.append(a)
                 if dedup is Dedup.OVERRIDEN:
                     pre_flush_set.add(a)
         for a in reversed(self.post):
@@ -140,12 +140,15 @@ class CompilerArgs(collections.abc.MutableSequence):
 
         #pre and post will overwrite every element that is in the container
         #only copy over args that are in _container but not in the post flush or pre flush set
-
+        if pre_flush_set or post_flush_set:
             for a in self._container:
                 if a not in post_flush_set and a not in pre_flush_set:
-                pre_flush.append(a)
+                    new.append(a)
+        else:
+            new.extend(self._container)
+        new.extend(post_flush)
 
-        self._container = list(pre_flush) + list(post_flush)
+        self._container = new
         self.pre.clear()
         self.post.clear()