Don't access last instruction of empty basicblocks

sweeneyde · sweeneyde · commit f224e2093dca · 2022-05-23T14:39:31.000-04:00
diff --git a/Python/compile.c b/Python/compile.c
@@ -7801,10 +7801,12 @@ scan_block_for_local(int target, basicblock *b, bool unsafe_to_start,
         if (b->b_next && !b->b_nofallthrough) {
             MAYBE_PUSH(b->b_next);
         }
-        struct instr *last = &b->b_instr[b->b_iused-1];
-        if (is_jump(last)) {
-            assert(last->i_target != NULL);
-            MAYBE_PUSH(last->i_target);
+        if (b->b_iused > 0) {
+            struct instr *last = &b->b_instr[b->b_iused-1];
+            if (is_jump(last)) {
+                assert(last->i_target != NULL);
+                MAYBE_PUSH(last->i_target);
+            }
         }
     }
 }

Original file line number	Diff line number	Diff line change
`@@ -7801,10 +7801,12 @@ scan_block_for_local(int target, basicblock *b, bool unsafe_to_start,`
`7801`	`7801`	`if (b->b_next && !b->b_nofallthrough) {`
`7802`	`7802`	`MAYBE_PUSH(b->b_next);`
`7803`	`7803`	`}`
`7804`		`- struct instr *last = &b->b_instr[b->b_iused-1];`
`7805`		`- if (is_jump(last)) {`
`7806`		`- assert(last->i_target != NULL);`
`7807`		`- MAYBE_PUSH(last->i_target);`
	`7804`	`+ if (b->b_iused > 0) {`
	`7805`	`+ struct instr *last = &b->b_instr[b->b_iused-1];`
	`7806`	`+ if (is_jump(last)) {`
	`7807`	`+ assert(last->i_target != NULL);`
	`7808`	`+ MAYBE_PUSH(last->i_target);`
	`7809`	`+ }`
`7808`	`7810`	`}`
`7809`	`7811`	`}`
`7810`	`7812`	`}`