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

1 files changed, 9 insertions, 2 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