fix: off by one error in forward jump calculation (python#19)

* fix: off by one error in forward jump calculation

* Test: fixed test for python#18

* test: remove broken test

---------

Co-authored-by: Jules <[email protected]>

Author: Fidget-Spinner

Python/tier2.c

@@ -1694,7 +1694,7 @@ _PyTier2_Code_DetectAndEmitBB(
         prev_type_guard->op.code == CHECK_LIST
     );
 #define END() goto end;
-#define JUMPBY(x) i += x + 1;
+#define JUMPBY(x) i += x;
 #define DISPATCH()        write_i = emit_i(write_i, specop, curr->op.arg); \
                           write_i = copy_cache_entries(write_i, curr+1, caches); \
                           i += caches; \
@@ -2067,8 +2067,14 @@ _PyTier2_Code_DetectAndEmitBB(
 
             // Jumps may be the end of a basic block if they are conditional (a branch).
             if (IS_JUMP_OPCODE(opcode)) {
+#if BB_DEBUG
+                fprintf(stderr, "Encountered a forward jump\n");
+#endif
                 // Unconditional forward jump... continue with the BB without writing the jump.
                 if (opcode == JUMP_FORWARD) {
+#if BB_DEBUG
+                    fprintf(stderr, "Encountered an unconditional forward jump\n");
+#endif
                     // JUMP offset (oparg) + current instruction + cache entries
                     JUMPBY(oparg);
                     continue;

tier2_test.py

@@ -462,5 +462,19 @@ def f(y,z,w):
 
     trigger_tier2(f, (1,1,1.))
 
+    # As long as it doesn't crash, everything's good.
+
+######################################################################
+# Tests for: Tier 2 unconditional forward jump                       #
+######################################################################
+with TestInfo("tier 2 unconditional forward jumps"):
+    # See https://github.com/pylbbv/pylbbv/issues/17 for more information.
+    def f(x):
+        for _ in [1]:
+            break
+        x+x # Force it to be optimisable
+
+    trigger_tier2(f, (1,))
+
     # As long as it doesn't crash, everything's good.
 print("Tests completed ^-^")