compiler: Avoid crash in private-append optimization

Avoid a crash in the private-append optimization due to attempting to
rewrite a literal which isn't a bitstring.

As value tracking is done without type information, it can happen that
when following def chains, we encounter literals which are not
bitstrings. We should never schedule a literal which isn't a bitstring
for rewriting when the complete literal is what we are tracking,
i.e. when the element is `self`.

Closes erlang#6847

lib/compiler/src/beam_ssa_private_append.erl

@@ -168,6 +168,13 @@ get_results(SSA, Element, Fun, DefSt) ->
 get_results([{_,#b_blk{last=#b_ret{arg=#b_var{}=V}}}|Rest],
             Acc, Element, Fun, DefSt) ->
     get_results(Rest, [{V,Element}|Acc], Element, Fun, DefSt);
+get_results([{_,#b_blk{last=#b_ret{arg=#b_literal{val=Lit}}}}|Rest],
+            Acc, Element=self, Fun, DefSt) when not is_bitstring(Lit) ->
+    %% As value tracking is done without type information, we can
+    %% follow def chains which don't terminate in a bitstring. This is
+    %% harmless, but we should ignore them and not, later on, try to
+    %% patch them to a bs_writable_binary.
+    get_results(Rest, Acc, Element, Fun, DefSt);
 get_results([{Lbl,#b_blk{last=#b_ret{arg=#b_literal{}}}}|Rest],
             Acc, Element, Fun, DefSt0) ->
     DefSt = add_literal(Fun, {ret,Lbl,Element}, DefSt0),

lib/compiler/test/beam_ssa_check_SUITE_data/private_append.erl

@@ -57,6 +57,8 @@
 	 transformable30/1,
 	 transformable31a/1,
 	 transformable31b/1,
+         transformable32/0,
+         transformable32/1,
 
 	 not_transformable1/2,
 	 not_transformable2/1,
@@ -748,6 +750,20 @@ transformable31([], Acc, a) when is_binary(Acc) ->
 transformable31([], Acc, b) when is_tuple(Acc) ->
     <<>>.
 
+%% Check that we don't crash (Github issue #6847) while attempting to
+%% patch the empty list, but also that the literal <<>> becomes a
+%% bs_init_writable.
+transformable32() ->
+    <<(transformable32(ok))/binary>>.
+
+transformable32(#{}) ->
+%ssa% (_) when post_ssa_opt ->
+%ssa% A = bs_init_writable(_),
+%ssa% ret(A).
+    [];
+transformable32(_) ->
+    <<>>.
+
 % Should not be transformed as we can't know the alias status of Acc
 not_transformable1([H|T], Acc) ->
 %ssa% (_, Arg1) when post_ssa_opt ->