fix: wrong offset in BB_BRANCH_IF_FLAG_SET codegen (python#21)

Author: JuliaPoo

Python/bytecodes.c

@@ -3355,10 +3355,10 @@ dummy_func(
                 _Py_CODEUNIT *t2_nextinstr = NULL;
                 _PyBBBranchCache *cache = (_PyBBBranchCache *)next_instr;
                 _Py_CODEUNIT *tier1_fallback = NULL;
-
                 t2_nextinstr = _PyTier2_GenerateNextBB(
                     frame, cache->bb_id_tagged, next_instr - 1,
-                    oparg, &tier1_fallback, bb_test);
+                //  v   We generate from the tier1 consequent BB, so offset (oparg) is 0.
+                    0, &tier1_fallback, bb_test);
                 if (t2_nextinstr == NULL) {
                     // Fall back to tier 1.
                     next_instr = tier1_fallback;

Python/generated_cases.c.h

@@ -1044,7 +1044,7 @@ const struct opcode_metadata _PyOpcode_opcode_metadata[256] = {
     [BB_BRANCH] = { true, INSTR_FMT_IBC },
     [BB_BRANCH_IF_FLAG_UNSET] = { true, INSTR_FMT_IBC },
     [BB_JUMP_IF_FLAG_UNSET] = { true, INSTR_FMT_IBC },
-    [BB_BRANCH_IF_FLAG_SET] = { true, INSTR_FMT_IBC },
+    [BB_BRANCH_IF_FLAG_SET] = { true, INSTR_FMT_IXC },
     [BB_JUMP_IF_FLAG_SET] = { true, INSTR_FMT_IBC },
     [BB_JUMP_BACKWARD_LAZY] = { true, INSTR_FMT_IB },
 };

Python/opcode_metadata.h

@@ -477,4 +477,20 @@ def f(x):
     trigger_tier2(f, (1,))
 
     # As long as it doesn't crash, everything's good.
+
+######################################################################
+# Tests for: Tier 2 BB_BRANCH_IF_FLAG_SET codegen                    #
+######################################################################
+with TestInfo("tier 2 BB_BRANCH_IF_FLAG_SET codegen"):
+    # See https://github.com/pylbbv/pylbbv/issues/18 for more information.
+    def f(x,l):
+        for _ in l:
+            _
+        x+x # Force it to be optimisable
+
+    trigger_tier2(f, (1, []))
+    f(1, [1])
+
+    # As long as it doesn't crash, everything's good.
+
 print("Tests completed ^-^")