Document how the pattern recognizer keeps all of its references in bounds.

Add a test in case the underlying assumptions ever change (i.e. the
compiler starts generating code blocks that are not punctuated by
RETURN_VALUE).

1 files changed, 8 insertions, 0 deletions

diff --git a/Python/compile.c b/Python/compile.c
index b458f7319a..6e68d0fd5d 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -677,6 +677,14 @@ optimize_code(PyObject *code, PyObject* consts, PyObject *names, PyObject *linen
 		goto exitUnchanged;
 	codestr = memcpy(codestr, PyString_AS_STRING(code), codelen);
 
+	/* Verify that RETURN_VALUE terminates the codestring.  This allows
+	   the various transformation patterns to look ahead several
+	   instructions without additional checks to make sure they are not
+	   looking beyond the end of the code string.
+	*/
+	if (codestr[codelen-1] != RETURN_VALUE)
+		goto exitUnchanged;
+
 	/* Mapping to new jump targets after NOPs are removed */
 	addrmap = PyMem_Malloc(codelen * sizeof(int));
 	if (addrmap == NULL)