Fix unsafe optimization of receives

Fix a problem where a receive marker would be set even if it
was not guaranteed that execution would reach a receive statement.

See the new test case `receive_SUITE:return_before_receive/1` for
an example where this would cause problems.

https://bugs.erlang.org/browse/ERL-1076

1 files changed, 7 insertions, 0 deletions

diff --git a/lib/compiler/src/beam_a.erl b/lib/compiler/src/beam_a.erl
index 0bccad1ecd..eadd858885 100644
--- a/lib/compiler/src/beam_a.erl
+++ b/lib/compiler/src/beam_a.erl
@@ -59,6 +59,13 @@ rename_instrs([{test,is_eq_exact,_,[Dst,Src]}=Test,
 rename_instrs([{test,is_eq_exact,_,[Same,Same]}|Is]) ->
     %% Same literal or same register. Will always succeed.
     rename_instrs(Is);
+rename_instrs([{recv_set,_},
+               {label,Lbl},
+               {loop_rec,{f,Fail},{x,0}},
+               {loop_rec_end,_},{label,Fail}|Is]) ->
+    %% This instruction sequence does nothing. All we need to
+    %% keep is the first label.
+    [{label,Lbl}|rename_instrs(Is)];
 rename_instrs([{loop_rec,{f,Fail},{x,0}},{loop_rec_end,_},{label,Fail}|Is]) ->
     %% This instruction sequence does nothing.
     rename_instrs(Is);