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bpo-41531: Fix compilation of dict literals with more than 0xFFFF elements #21850

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Aug 13, 2020
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bpo-41531: Fix compilation of dict literals with more than 0xFFFF elements #21850

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10 changes: 10 additions & 0 deletions Lib/test/test_compile.py
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Original file line number Diff line number Diff line change
Expand Up @@ -752,6 +752,16 @@ def continue_in_while():
self.assertEqual(None, opcodes[0].argval)
self.assertEqual('RETURN_VALUE', opcodes[1].opname)

def test_big_dict_literal(self):
# The compiler has a flushing point in "compiler_dict" that calls compiles
# a portion of the dictionary literal when the loop that iterates over the items
# reaches 0xFFFF elements but the code was not including the boundary element,
# dropping the key at position 0xFFFF. See bpo-41531 for more information

dict_size = 0xFFFF + 1
the_dict = "{" + ",".join(f"{x}:{x}" for x in range(dict_size)) + "}"
self.assertEqual(len(eval(the_dict)), dict_size)

class TestExpressionStackSize(unittest.TestCase):
# These tests check that the computed stack size for a code object
# stays within reasonable bounds (see issue #21523 for an example
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2020-08-12-19-32-15.bpo-41531.WgPzjT.rst
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@@ -0,0 +1,2 @@
Fix a bug that was dropping keys when compiling dict literals with more than
0xFFFF elements. Patch by Pablo Galindo.
2 changes: 1 addition & 1 deletion Python/compile.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3894,7 +3894,7 @@ compiler_dict(struct compiler *c, expr_ty e)
}
else {
if (elements == 0xFFFF) {
if (!compiler_subdict(c, e, i - elements, i)) {
if (!compiler_subdict(c, e, i - elements, i + 1)) {
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@pablogsal pablogsal Aug 12, 2020 •
edited
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I just made the minimum edit to fix the problem but I would just remove this checkpoint and simply keep incrementing elements and leave the last check fill the dict, which is simpler and less error prone. Any reason we want to flush when we are in USHRT_MAX elements @markshannon ?

To be clear, this is what I propose:

diff --git a/Python/compile.c b/Python/compile.c
index b2beef9327..a348b13676 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -3893,19 +3893,7 @@ compiler_dict(struct compiler *c, expr_ty e)
             ADDOP_I(c, DICT_UPDATE, 1);
         }
         else {
-            if (elements == USHRT_MAX) {
-                if (!compiler_subdict(c, e, i - elements, i + 1)) {
-                    return 0;
-                }
-                if (have_dict) {
-                    ADDOP_I(c, DICT_UPDATE, 1);
-                }
-                have_dict = 1;
-                elements = 0;
-            }
-            else {
                 elements++;
-            }
         }
     }
     if (elements) {

return 0;
}
if (have_dict) {
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