compiler: Correct live calculation when get/1 is used in try...catch

In the following code excerpt, the instruction marked below was
incorrectly removed:
    .
    .
    .
    {'try',{y,2},{f,TryCaseLabel}}.
    {bif,get,{f,0},[{x,0}],{x,0}}.
    {move,{x,1},{y,0}}.
    {move,{x,3},{y,1}}.             <======= Incorrectly removed
    {jump,{f,TryEndLabel}}.

  {label,TryEndLabel}.
    {try_end,{y,2}}.
    {deallocate,3}.
    return.

  {label,TryCaseLabel}.
    {try_case,{y,2}}.
    .
    .
    .

beam_utils indicated that {y,1} was not used at TryEndLabel,
which by itself is correct. But it is still not safe to remove
the instruction, because {y,1} might be used at TryCaseLabel
if an exception occurs.

Noticed-by: Eric Merritt

2 files changed, 25 insertions, 4 deletions

diff --git a/lib/compiler/src/beam_utils.erl b/lib/compiler/src/beam_utils.erl
index a631b8cd69..116ede0bc9 100644
--- a/lib/compiler/src/beam_utils.erl
+++ b/lib/compiler/src/beam_utils.erl
@@ -474,8 +474,15 @@ check_liveness(R, [{make_fun2,_,_,_,NumFree}|Is], St) ->
     end;
 check_liveness(R, [{try_end,Y}|Is], St) ->
     case R of
-	Y -> {killed,St};
-	_ -> check_liveness(R, Is, St)
+	Y ->
+	    {killed,St};
+	{y,_} ->
+	    %% y registers will be used if an exception occurs and
+	    %% control transfers to the label given in the previous
+	    %% try/2 instruction.
+	    {used,St};
+	_ ->
+	    check_liveness(R, Is, St)
     end;
 check_liveness(R, [{catch_end,Y}|Is], St) ->
     case R of
diff --git a/lib/compiler/test/trycatch_SUITE.erl b/lib/compiler/test/trycatch_SUITE.erl
index 760cf17225..09a23724fe 100644
--- a/lib/compiler/test/trycatch_SUITE.erl
+++ b/lib/compiler/test/trycatch_SUITE.erl
@@ -24,7 +24,7 @@
 	 catch_oops/1,after_oops/1,eclectic/1,rethrow/1,
 	 nested_of/1,nested_catch/1,nested_after/1,
 	 nested_horrid/1,last_call_optimization/1,bool/1,
-	 plain_catch_coverage/1,andalso_orelse/1]).
+	 plain_catch_coverage/1,andalso_orelse/1,get_in_try/1]).
 
 -include_lib("test_server/include/test_server.hrl").
 
@@ -35,7 +35,7 @@ all() ->
     [basic, lean_throw, try_of, try_after, catch_oops,
      after_oops, eclectic, rethrow, nested_of, nested_catch,
      nested_after, nested_horrid, last_call_optimization,
-     bool, plain_catch_coverage, andalso_orelse].
+     bool, plain_catch_coverage, andalso_orelse, get_in_try].
 
 groups() -> 
     [].
@@ -928,3 +928,17 @@ andalso_orelse_2({Type,Keyval}) ->
 
 zero() ->
     0.0.
+
+get_in_try(_) ->
+    undefined = get_valid_line([a], []),
+    ok.
+
+get_valid_line([_|T]=Path, Annotations) ->
+    try
+        get(Path)
+	%% beam_dead used to optimize away an assignment to {y,1}
+	%% because it didn't appear to be used.
+    catch
+        _:not_found ->
+            get_valid_line(T, Annotations)
+    end.