beam_reorder: Eliminate compiler crash

c288ab87 added beam_reorder to move get_tuple_element instructions.

Compiling code such as the following would crash the compiler:

  alloc(_U1, _U2, R) ->
    V = R#alloc.version,
    Res = id(V),
    _ = id(0),
    Res.

The crash would occur because the following two instructions:

    {get_tuple_element,{x,2},1,{x,1}}.
    {allocate_zero,1,2}.

were swapped and rewritten to:

    {allocate_zero,1,1}.
    {get_tuple_element,{x,2},1,{x,1}}.

That transformation is not safe because the allocate_zero instruction
would kill {x,2}, which is the register that is holding the reference
to the tuple. Only do the transformation when the tuple reference is
in an x register with a lower number than the destination register.

1 files changed, 13 insertions, 8 deletions

diff --git a/lib/compiler/src/beam_reorder.erl b/lib/compiler/src/beam_reorder.erl
index 70adca6b04..41586a7bf2 100644
--- a/lib/compiler/src/beam_reorder.erl
+++ b/lib/compiler/src/beam_reorder.erl
@@ -110,14 +110,19 @@ reorder_1([{test,_,{f,L},Ss}=I|Is0], D0,
 		    end
 	    end
     end;
-reorder_1([{allocate_zero,N,Live}|Is], D,
-	  [{get_tuple_element,_,_,{x,X}}=G|Acc])
-  when X+1 =:= Live ->
-    %% Move allocation instruction upwards past get_tuple_element
-    %% instructions to give more opportunities for moving
-    %% get_tuple_element instructions.
-    I = {allocate_zero,N,X},
-    reorder_1([I,G|Is], D, Acc);
+reorder_1([{allocate_zero,N,Live}=I0|Is], D,
+	  [{get_tuple_element,{x,Tup},_,{x,Dst}}=G|Acc]=Acc0) ->
+    case Tup < Dst andalso Dst+1 =:= Live of
+	true ->
+	    %% Move allocation instruction upwards past
+	    %% get_tuple_element instructions to create more
+	    %% opportunities for moving get_tuple_element
+	    %% instructions.
+	    I = {allocate_zero,N,Dst},
+	    reorder_1([I,G|Is], D, Acc);
+	false ->
+	    reorder_1(Is, D, [I0|Acc0])
+    end;
 reorder_1([I|Is], D, Acc) ->
     reorder_1(Is, D, [I|Acc]);
 reorder_1([], _, Acc) -> reverse(Acc).