4 files changed, 26 insertions, 19 deletions

diff --git a/erts/emulator/beam/beam_emu.c b/erts/emulator/beam/beam_emu.c
index bc95ccec52..ef9abcde08 100644
--- a/erts/emulator/beam/beam_emu.c
+++ b/erts/emulator/beam/beam_emu.c
@@ -1454,6 +1454,7 @@ handle_error(Process* c_p, BeamInstr* pc, Eterm* reg, ErtsCodeMFA *bif_mfa)
 	reg[3] = c_p->ftrace;
         if ((new_pc = next_catch(c_p, reg))) {
 	    c_p->cp = 0;	/* To avoid keeping stale references. */
+            c_p->msg.saved_last = 0;  /* No longer safe to use this position */
 	    return new_pc;
 	}
 	if (c_p->catches > 0) erts_exit(ERTS_ERROR_EXIT, "Catch not found");
diff --git a/erts/emulator/beam/erl_message.h b/erts/emulator/beam/erl_message.h
index 9c8cf84e43..a14f4f51d8 100644
--- a/erts/emulator/beam/erl_message.h
+++ b/erts/emulator/beam/erl_message.h
@@ -167,10 +167,9 @@ typedef struct {
     Sint len;            /* queue length */
 
     /*
-     * The following two fields are used by the recv_mark/1 and
+     * The following field is used by the recv_mark/1 and
      * recv_set/1 instructions.
      */
-    BeamInstr* mark;		/* address to rec_loop/2 instruction */
     ErtsMessage** saved_last;	/* saved last pointer */
 } ErlMessageQueue;
 
@@ -236,12 +235,17 @@ typedef struct erl_trace_message_queue__ {
      (p)->msg.len--; \
      if (__mp == NULL) \
          (p)->msg.last = (p)->msg.save; \
-     (p)->msg.mark = 0; \
 } while(0)
 
-/* Reset message save point (after receive match) */
-#define JOIN_MESSAGE(p) \
-     (p)->msg.save = &(p)->msg.first
+/*
+ * Reset message save point (after receive match).
+ * Also invalidate the saved position since it may no
+ * longer be safe to use.
+ */
+#define JOIN_MESSAGE(p) do {                    \
+    (p)->msg.save = &(p)->msg.first;            \
+    (p)->msg.saved_last = 0;                    \
+} while(0)
 
 /* Save current message */
 #define SAVE_MESSAGE(p) \
diff --git a/erts/emulator/beam/msg_instrs.tab b/erts/emulator/beam/msg_instrs.tab
index 8055a8616f..d6d4d2fb49 100644
--- a/erts/emulator/beam/msg_instrs.tab
+++ b/erts/emulator/beam/msg_instrs.tab
@@ -43,27 +43,23 @@
 //  *
 //  */
 
-recv_mark(Dest) {
+i_recv_mark() {
     /*
-     * Save the current position in message buffer and the
-     * the label for the loop_rec/2 instruction for the
-     * the receive statement.
+     * Save the current position in message buffer.
      */
-    $SET_REL_I(c_p->msg.mark, $Dest);
     c_p->msg.saved_last = c_p->msg.last;
 }
 
 i_recv_set() {
     /*
-     * If the mark is valid (points to the loop_rec/2
-     * instruction that follows), we know that the saved
-     * position points to the first message that could
-     * possibly be matched out.
+     * If c_p->msg.saved_last is non-zero, it points to the first
+     * message that could possibly be matched out.
      *
-     * If the mark is invalid, we do nothing, meaning that
-     * we will look through all messages in the message queue.
+     * If c_p->msg.saved_last is zero, it means that it was invalidated
+     * because another receive was executed before this i_recv_set()
+     * instruction was reached.
      */
-    if (c_p->msg.mark == (BeamInstr *) ($NEXT_INSTRUCTION)) {
+    if (c_p->msg.saved_last) {
         c_p->msg.save = c_p->msg.saved_last;
     }
     SET_I($NEXT_INSTRUCTION);
@@ -131,6 +127,7 @@ i_loop_rec(Dest) {
             ASSERT(HTOP == c_p->htop && E == c_p->stop);
             /* TODO: Add DTrace probe for this bad message situation? */
             UNLINK_MESSAGE(c_p, msgp);
+            c_p->msg.saved_last = 0; /* Better safe than sorry. */
             msgp->next = NULL;
             erts_cleanup_messages(msgp);
             goto loop_rec__;
diff --git a/erts/emulator/beam/ops.tab b/erts/emulator/beam/ops.tab
index a560bde920..3df91056cb 100644
--- a/erts/emulator/beam/ops.tab
+++ b/erts/emulator/beam/ops.tab
@@ -1565,7 +1565,12 @@ on_load
 #
 # R14A.
 #
-recv_mark f
+# Modified in OTP 21 because it turns out that we don't need the
+# label after all.
+#
+
+recv_mark f => i_recv_mark
+i_recv_mark
 
 recv_set Fail | label Lbl | loop_rec Lf Reg => \
    i_recv_set | label Lbl | loop_rec Lf Reg