ZJIT: Add comments about zjit_frame_active

Author: k0kubun

vm.c

@@ -2892,7 +2892,18 @@ zjit_materialize_frames(const rb_execution_context_t *ec, rb_control_frame_t *cf
     VM_ASSERT(cfp <= end_cfp);
 
     while (true) {
+        // If materialize_target is false, we skip materializing ec->tag->cfp.
+        //
+        // When JIT code calls a C function that does the same number of setjmps and
+        // longjmps, e.g. rb_hash_aref, it calls zjit_materialize_frames but goes
+        // back to the JIT code. In that case, we don't want to materialize the frame
+        // and clear cfp->jit_return, which will still be used by the JIT code.
+        //
+        // When JIT code calls a C function that does more longjmps than setjmps,
+        // it would not go back to the JIT code. So ec->tag->cfp should be materialized
+        // in that case.
         if (cfp == end_cfp && !materialize_target) break;
+
         if (CFP_ZJIT_FRAME_P(cfp)) {
             const zjit_jit_frame_t *jit_frame = CFP_ZJIT_FRAME(cfp);
             cfp->pc = jit_frame->pc;

vm_core.h

@@ -1030,6 +1030,10 @@ struct rb_vm_tag {
     // ec->cfp as of EC_PUSH_TAG, which is saved for materializing JITFrame.
     rb_control_frame_t *cfp;
     // Whether cfp had a ZJIT frame before this tag's setjmp was established.
+    // It's used for checking if zjit_materialize_frames should materialize
+    // the frame or not when the tag is popped. If zjit_frame_active is true,
+    // we don't want to materialize cfp->jit_return, which will still be used
+    // by JIT code.
     bool zjit_frame_active;
 #endif
 };