bpo-26415: reduce peak memory consumption by the parser

Compress some branchless stretches in the parse tree into a single node.
AST is not affected.

Author: tyomitch

.gitattributes

@@ -59,3 +59,4 @@ Doc/library/token-list.inc  linguist-generated=true
 Include/token.h             linguist-generated=true
 Lib/token.py                linguist-generated=true
 Parser/token.c              linguist-generated=true
+Modules/gramvalid.c         linguist-generated=true

Include/node.h

@@ -22,6 +22,7 @@ PyAPI_FUNC(node *) PyNode_New(int type);
 PyAPI_FUNC(int) PyNode_AddChild(node *n, int type,
                                 char *str, int lineno, int col_offset,
                                 int end_lineno, int end_col_offset);
+PyAPI_FUNC(void) PyNode_Compress(node *n);
 PyAPI_FUNC(void) PyNode_Free(node *n);
 #ifndef Py_LIMITED_API
 PyAPI_FUNC(Py_ssize_t) _PyNode_SizeOf(node *n);

Makefile.pre.in

@@ -794,9 +794,11 @@ regen-grammar: regen-token
 	$(PYTHON_FOR_REGEN) -m Parser.pgen $(srcdir)/Grammar/Grammar \
 		$(srcdir)/Grammar/Tokens \
 		$(srcdir)/Include/graminit.h.new \
-		$(srcdir)/Python/graminit.c.new
+		$(srcdir)/Python/graminit.c.new \
+		$(srcdir)/Modules/gramvalid.c.new
 	$(UPDATE_FILE) $(srcdir)/Include/graminit.h $(srcdir)/Include/graminit.h.new
 	$(UPDATE_FILE) $(srcdir)/Python/graminit.c $(srcdir)/Python/graminit.c.new
+	$(UPDATE_FILE) $(srcdir)/Modules/gramvalid.c $(srcdir)/Modules/gramvalid.c.new
 
 .PHONY=regen-ast
 regen-ast:

Misc/NEWS.d/next/Core and Builtins/2018-12-06-16-55-19.bpo-26415.jbufv5.rst

@@ -0,0 +1,3 @@
+Reduce peak memory consumption by the parser, by compressing some
+branchless stretches in the parse tree into a single node. AST is
+not affected.