[3.11] gh-104615: don't make unsafe swaps in apply_static_swaps (GH-104620). (#104636)

(cherry picked from commit 0589c6a4d3d822cace42050198cb9a5e99c879ad)

1 files changed, 20 insertions, 0 deletions

diff --git a/Python/compile.c b/Python/compile.c
index f87a423..1c712fb 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -8690,6 +8690,9 @@ swaptimize(basicblock *block, int *ix)
 #define SWAPPABLE(opcode) \
     ((opcode) == STORE_FAST || (opcode) == POP_TOP)
 
+#define STORES_TO(instr) \
+    (((instr).i_opcode == STORE_FAST) ? (instr).i_oparg : -1)
+
 static int
 next_swappable_instruction(basicblock *block, int i, int lineno)
 {
@@ -8741,6 +8744,23 @@ apply_static_swaps(basicblock *block, int i)
                 return;
             }
         }
+        // The reordering is not safe if the two instructions to be swapped
+        // store to the same location, or if any intervening instruction stores
+        // to the same location as either of them.
+        int store_j = STORES_TO(block->b_instr[j]);
+        int store_k = STORES_TO(block->b_instr[k]);
+        if (store_j >= 0 || store_k >= 0) {
+            if (store_j == store_k) {
+                return;
+            }
+            for (int idx = j + 1; idx < k; idx++) {
+                int store_idx = STORES_TO(block->b_instr[idx]);
+                if (store_idx >= 0 && (store_idx == store_j || store_idx == store_k)) {
+                    return;
+                }
+            }
+        }
+
         // Success!
         swap->i_opcode = NOP;
         struct instr temp = block->b_instr[j];