refcount_insertion: add RC_DIFF differential logging + RC_USE_CPP switch

SonicField · SonicField · commit cabb1001a074 · 2026-04-18T11:58:48.000-07:00
RC_DIFF=1: logs every Incref/Decref with register ID and block ID
  - C pass: "C +ref vN bbM" / "C -ref vN bbM"
  - C++ pass: "CPP +ref vN bbM" / "CPP -ref vN bbM"
RC_USE_CPP=1: forces C++ pass instead of C pass (for A/B comparison)

Usage: run same test with both passes, sort output, diff.
First divergence on alternating_sum: v47 Decref in bb2 (C) vs bb1 (C++).
diff --git a/Python/jit/hir/refcount_insertion.cpp b/Python/jit/hir/refcount_insertion.cpp
@@ -490,9 +490,18 @@ bool isUncounted(const Register* reg) {
 }
 
 // Insert an Incref of `reg` before `cursor`.
+static int g_cpp_rc_log = -1;
+static int cpp_rc_log_enabled() {
+  if (g_cpp_rc_log < 0) g_cpp_rc_log = getenv("RC_DIFF") != nullptr;
+  return g_cpp_rc_log;
+}
+
 void insertIncref(Env& env, Register* reg, Instr& cursor) {
   JIT_DCHECK(env.mutate, "Attempt to insert incref with mutate == false");
   JIT_DCHECK(!isUncounted(reg), "Attempt to incref an uncounted value");
+  if (cpp_rc_log_enabled()) {
+    fprintf(stderr, "CPP +ref v%d bb%d\n", reg->id(), cursor.block()->id);
+  }
   Instr* incref;
   if (reg->type() <= TObject) {
     incref = static_cast<Instr*>(hir_c_create_incref(reg));
@@ -512,6 +521,9 @@ void insertIncref(Env& env, Register* reg, Instr& cursor) {
 void insertDecref(Env& env, Register* reg, Instr& cursor) {
   JIT_DCHECK(env.mutate, "Attempt to insert decref with mutate == false");
   JIT_DCHECK(!isUncounted(reg), "Attempt to decref an uncounted value");
+  if (cpp_rc_log_enabled()) {
+    fprintf(stderr, "CPP -ref v%d bb%d\n", reg->id(), cursor.block()->id);
+  }
   Instr* decref;
   if (reg->type() <= TObject) {
     decref = static_cast<Instr*>(hir_c_create_decref(reg));
@@ -1305,12 +1317,14 @@ void RefcountInsertion::Run(Function& func) {
   bindGuards(func);
   func.cfg.splitCriticalEdges();
 
-  PhxRefcountEnv *c_env = phx_rc_env_create(static_cast<void*>(&func));
-  phx_rc_run(c_env);
-  phx_rc_env_destroy(c_env);
-  removeTrampolineBlocks(&func.cfg);
-  optimizeLongDecrefRuns(func);
-  return;
+  if (!getenv("RC_USE_CPP")) {
+    PhxRefcountEnv *c_env = phx_rc_env_create(static_cast<void*>(&func));
+    phx_rc_run(c_env);
+    phx_rc_env_destroy(c_env);
+    removeTrampolineBlocks(&func.cfg);
+    optimizeLongDecrefRuns(func);
+    return;
+  }
 
   TRACE(
       "Starting refcount insertion for '{}':\n{}",
diff --git a/Python/jit/hir/refcount_pass_c.c b/Python/jit/hir/refcount_pass_c.c
@@ -12,15 +12,27 @@
 #include "cinderx/Common/jit_log_c.h"
 #include "Python.h"
 
+#include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 
+static int g_c_rc_log = -1;
+static int c_rc_log_enabled(void) {
+    if (g_c_rc_log < 0) g_c_rc_log = getenv("RC_DIFF") != NULL;
+    return g_c_rc_log;
+}
+
 /* AliasClass bits for managed heap (AManagedHeapAny) */
 #define ALIAS_MANAGED_HEAP_ANY 0x3FC
 
 /* Bridge: check if register type is subtype of TObject (needs C++ constant) */
 extern int phx_rc_reg_is_object(void *reg);
 
 void phx_rc_insert_incref(PhxRefcountEnv *env, void *reg, void *cursor) {
+    if (env->mutate && c_rc_log_enabled()) {
+        const HirBasicBlock *blk = (const HirBasicBlock *)((HirInstrLayout *)cursor)->block;
+        fprintf(stderr, "C +ref v%d bb%d\n", hir_reg_id(reg), hir_bb_id(blk));
+    }
     void *incref = phx_rc_reg_is_object(reg)
         ? hir_c_create_incref(reg)
         : hir_c_create_xincref(reg);
@@ -30,6 +42,10 @@ void phx_rc_insert_incref(PhxRefcountEnv *env, void *reg, void *cursor) {
 }
 
 void phx_rc_insert_decref(PhxRefcountEnv *env, void *reg, void *cursor) {
+    if (env->mutate && c_rc_log_enabled()) {
+        const HirBasicBlock *blk = (const HirBasicBlock *)((HirInstrLayout *)cursor)->block;
+        fprintf(stderr, "C -ref v%d bb%d\n", hir_reg_id(reg), hir_bb_id(blk));
+    }
     void *decref = phx_rc_reg_is_object(reg)
         ? hir_c_create_decref(reg)
         : hir_c_create_xdecref(reg);
