compiler: Correct liveness optimization for wait/1

The live optimization in beam_utils:live_opt/4 did not take into
account that the wait/1 instruction *never* falls through to
the next instruction (it has the same effect on the control flow
as the jump/1 instruction).

2 files changed, 21 insertions, 4 deletions

diff --git a/lib/compiler/src/beam_utils.erl b/lib/compiler/src/beam_utils.erl
index 554c14f57a..e623bcc6a5 100644
--- a/lib/compiler/src/beam_utils.erl
+++ b/lib/compiler/src/beam_utils.erl
@@ -734,6 +734,8 @@ live_opt([{loop_rec,_Fail,_Dst}=I|Is], _, D, Acc) ->
     live_opt(Is, 0, D, [I|Acc]);
 live_opt([timeout=I|Is], _, D, Acc) ->
     live_opt(Is, 0, D, [I|Acc]);
+live_opt([{wait,_}=I|Is], _, D, Acc) ->
+    live_opt(Is, 0, D, [I|Acc]);
 
 %% Transparent instructions - they neither use nor modify x registers.
 live_opt([{deallocate,_}=I|Is], Regs, D, Acc) ->
@@ -744,8 +746,6 @@ live_opt([{try_end,_}=I|Is], Regs, D, Acc) ->
     live_opt(Is, Regs, D, [I|Acc]);
 live_opt([{loop_rec_end,_}=I|Is], Regs, D, Acc) ->
     live_opt(Is, Regs, D, [I|Acc]);
-live_opt([{wait,_}=I|Is], Regs, D, Acc) ->
-    live_opt(Is, Regs, D, [I|Acc]);
 live_opt([{wait_timeout,_,{Tag,_}}=I|Is], Regs, D, Acc) when Tag =/= x ->
     live_opt(Is, Regs, D, [I|Acc]);
 live_opt([{line,_}=I|Is], Regs, D, Acc) ->
diff --git a/lib/compiler/test/receive_SUITE.erl b/lib/compiler/test/receive_SUITE.erl
index e60584d4ab..ec49267ded 100644
--- a/lib/compiler/test/receive_SUITE.erl
+++ b/lib/compiler/test/receive_SUITE.erl
@@ -23,7 +23,8 @@
 -export([all/0, suite/0,groups/0,init_per_suite/1, end_per_suite/1, 
 	 init_per_group/2,end_per_group/2,
 	 init_per_testcase/2,end_per_testcase/2,
-	 export/1,recv/1,coverage/1,otp_7980/1,ref_opt/1]).
+	 export/1,recv/1,coverage/1,otp_7980/1,ref_opt/1,
+	 wait/1]).
 
 -include_lib("test_server/include/test_server.hrl").
 
@@ -44,7 +45,7 @@ all() ->
 
 groups() -> 
     [{p,test_lib:parallel(),
-      [recv,coverage,otp_7980,ref_opt,export]}].
+      [recv,coverage,otp_7980,ref_opt,export,wait]}].
 
 
 init_per_suite(Config) ->
@@ -252,4 +253,20 @@ export_1(Reference) ->
     id({build,self()}),
     Result.
 
+wait(Config) when is_list(Config) ->
+    self() ! <<42>>,
+    <<42>> = wait_1(r, 1, 2),
+    {1,2,3} = wait_1(1, 2, 3),
+    ok.
+
+wait_1(r, _, _) ->
+    receive
+	B when byte_size(B) > 0 ->
+	    B
+    end;
+%% beam_utils would wrongly assume that wait/1 could fall through
+%% to the next clause.
+wait_1(A, B, C) ->
+    {A,B,C}.
+
 id(I) -> I.