beam_block: Do optimizations in the safe order

Moving of allocation instructions upwards in the instruction
stream (in order to enable further optimizations) in beam_block,
is implemented with the assumption that if a register {x,X}
contains a valid term, then all other x register with lower
numbers than X also contain valid terms. That assumption is
true after code generation.

The beam_utils:live_opt/1 optimization, however, may invalidate
that assumption. For instance, if a receive statement exports a
variable that is used, but the return value of the receive statement
is not used, then {x,1} but not {x,0} contains a valid term at the
end of the receive statement. If the receive statement is
followed by

    {bif,self,{f,0},[],{x,0}}.
    {test_heap,NumberOfWords,2}.

moving the allocation upwards will produce

    {test_heap,NumberOfWords,2}.
    {bif,self,{f,0},[],{x,0}}.

which will cause the beam_validator pass to scream loudly that
{x,0} is not live at the test_heap instruction.

Fix the problem by doing the optimizations in reverse order.

Reported-by: Jim Engquist

1 files changed, 23 insertions, 2 deletions

diff --git a/lib/compiler/test/receive_SUITE.erl b/lib/compiler/test/receive_SUITE.erl
index fca3f0387b..2a592dd669 100644
--- a/lib/compiler/test/receive_SUITE.erl
+++ b/lib/compiler/test/receive_SUITE.erl
@@ -21,7 +21,7 @@
 -module(receive_SUITE).
 
 -export([all/1,init_per_testcase/2,fin_per_testcase/2,
-	 recv/1,coverage/1,otp_7980/1,ref_opt/1]).
+	 recv/1,coverage/1,otp_7980/1,ref_opt/1,export/1]).
 
 -include("test_server.hrl").
 
@@ -36,7 +36,7 @@ fin_per_testcase(_Case, Config) ->
 
 all(suite) ->
     test_lib:recompile(?MODULE),
-    [recv,coverage,otp_7980,ref_opt].
+    [recv,coverage,otp_7980,ref_opt,export].
 
 -record(state, {ena = true}).
 
@@ -205,4 +205,25 @@ collect_recv_opt_instrs(Code) ->
 		end] || {function,_,_,_,Is} <- Code],
     lists:append(L).
 
+export(Config) when is_list(Config) ->
+    Ref = make_ref(),
+    ?line self() ! {result,Ref,42},
+    ?line 42 = export_1(Ref),
+    ?line {error,timeout} = export_1(Ref),
+    ok.
+
+export_1(Reference) ->
+    id(Reference),
+    receive
+	{result,Reference,Result} ->
+	    Result
+    after 1 ->
+	    Result = {error,timeout}
+    end,
+    %% Result ({x,1}) is used, but not the return value ({x,0})
+    %% of the receive. Used to be incorrectly optimized
+    %% by beam_block.
+    id({build,self()}),
+    Result.
+
 id(I) -> I.