Merge branch 'bjorn/compiler/dup-bug-fix/OTP-12453'

* bjorn/compiler/dup-bug-fix/OTP-12453:
  Teach case_opt/3 to avoid unnecessary building
  sys_core_fold: Optimize let statements more aggressively
  Suppress warnings for expressions that are assigned to '_'
  trace_bif_SUITE: Ensure that a call to time/0 is not removed

7 files changed, 239 insertions, 107 deletions

diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl
index 604c65f9b3..de92792a68 100644
--- a/lib/compiler/src/sys_core_fold.erl
+++ b/lib/compiler/src/sys_core_fold.erl
@@ -680,23 +680,15 @@ count_bits_1(Int, Bits) -> count_bits_1(Int bsr 64, Bits+64).
 %%  a rewritten expression consisting of a sequence of
 %%  the arguments only is returned.
 
-useless_call(effect, #c_call{anno=Anno,
-			     module=#c_literal{val=Mod},
+useless_call(effect, #c_call{module=#c_literal{val=Mod},
 			     name=#c_literal{val=Name},
 			     args=Args}=Call) ->
     A = length(Args),
     case erl_bifs:is_safe(Mod, Name, A) of
 	false ->
 	    case erl_bifs:is_pure(Mod, Name, A) of
-		true ->
-		    case member(result_not_wanted, Anno) of
-			false ->
-			    add_warning(Call, result_ignored);
-			true ->
-			    ok
-		    end;
-		false ->
-		    ok
+		true -> add_warning(Call, result_ignored);
+		false -> ok
 	    end,
 	    no;
 	true ->
@@ -2038,12 +2030,31 @@ case_opt_args([], Cs, _Sub, _LitExpr, Acc) ->
 %%  Try to expand one argument to several arguments (if tuple/list)
 %%  or to remove a literal argument.
 %%
-case_opt_arg(E0, Sub, Cs0, LitExpr) ->
-    E = maybe_replace_var(E0, Sub),
-    case cerl:is_data(E) of
+case_opt_arg(E0, Sub, Cs, LitExpr) ->
+    case cerl:is_c_var(E0) of
+	false ->
+	    case_opt_arg_1(E0, Cs, LitExpr);
+	true ->
+	    case case_will_var_match(Cs) of
+		true ->
+		    %% All clauses will match a variable in the
+		    %% current position. Don't expand this variable
+		    %% (that can only make the code worse).
+		    {error,Cs};
+		false ->
+		    %% If possible, expand this variable to a previously
+		    %% matched term.
+		    E = case_expand_var(E0, Sub),
+		    case_opt_arg_1(E, Cs, LitExpr)
+	    end
+    end.
+
+case_opt_arg_1(E0, Cs0, LitExpr) ->
+    case cerl:is_data(E0) of
 	false ->
             {error,Cs0};
 	true ->
+	    E = case_opt_compiler_generated(E0),
 	    Cs = case_opt_nomatch(E, Cs0, LitExpr),
 	    case cerl:data_type(E) of
 		{atomic,_} ->
@@ -2053,18 +2064,42 @@ case_opt_arg(E0, Sub, Cs0, LitExpr) ->
 	    end
     end.
 
-%% maybe_replace_var(Expr0, Sub) -> Expr
+%% case_will_var_match([Clause]) -> true | false.
+%%  Return if all clauses will match a variable in the
+%%  current position.
+%%
+case_will_var_match(Cs) ->
+    all(fun({[P|_],_,_,_}) ->
+		case cerl_clauses:match(P, any) of
+		    {true,_} -> true;
+		    _ -> false
+		end
+	end, Cs).
+
+
+%% case_opt_compiler_generated(Core) -> Core'
+%%  Mark Core expressions as compiler generated to ensure that
+%%  no warnings are generated if they turn out to be unused.
+%%  To pretty-printed Core Erlang easier to read, don't mark
+%%  constructs that can't cause warnings to be emitted.
+%%
+case_opt_compiler_generated(Core) ->
+    F = fun(C) ->
+		case cerl:type(C) of
+		    alias -> C;
+		    var -> C;
+		    _ -> cerl:set_ann(C, [compiler_generated])
+		end
+	end,
+    cerl_trees:map(F, Core).
+
+
+%% case_expand_var(Expr0, Sub) -> Expr
 %%  If Expr0 is a variable that has been previously matched and
 %%  is known to be a tuple, return the tuple instead. Otherwise
 %%  return Expr0 unchanged.
 %%
-maybe_replace_var(E, Sub) ->
-    case cerl:is_c_var(E) of
-	false -> E;
-	true -> maybe_replace_var_1(E, Sub)
-    end.
-
-maybe_replace_var_1(E, #sub{t=Tdb}) ->
+case_expand_var(E, #sub{t=Tdb}) ->
     case orddict:find(cerl:var_name(E), Tdb) of
 	{ok,T0} ->
 	    case cerl:is_c_tuple(T0) of
@@ -2081,9 +2116,8 @@ maybe_replace_var_1(E, #sub{t=Tdb}) ->
 		    %% operator will fail when used in map
 		    %% construction (only the '=>' operator is allowed
 		    %% when constructing a map from scratch).
-		    ToData = fun coerce_to_data/1,
 		    try
-			cerl_trees:map(ToData, T0)
+			cerl_trees:map(fun coerce_to_data/1, T0)
 		    catch
 			throw:impossible ->
 			    %% Something unsuitable was found (map or
@@ -2137,8 +2171,9 @@ case_opt_nomatch(_, [], _) -> [].
 %%  will match, and we can remove the corresponding pattern from
 %%  each clause.
 %%
-%%  The only complication is if the literal is a binary. Binary
-%%  pattern matching is tricky, so we will give up in that case.
+%%  The only complication is if the literal is a binary or map.
+%%  In general, it is difficult to know whether a binary or
+%%  map pattern will match, so we give up in that case.
 
 case_opt_lit(Lit, Cs0) ->
     try case_opt_lit_1(Lit, Cs0) of
@@ -2165,6 +2200,10 @@ case_opt_lit_1(E, [{[P|Ps],C,PsAcc,Bs0}|Cs]) ->
 case_opt_lit_1(_, []) -> [].
 
 %% case_opt_data(Expr, Clauses0, LitExpr) -> {ok,Exprs,Clauses}
+%%  The case expression is a non-atomic data constructor (cons
+%%  or tuple). We can know at compile time whether each clause
+%%  will match, and we can delay the building of the data to
+%%  the clauses where it is actually needed.
 
 case_opt_data(E, Cs0) ->
     Es = cerl:data_es(E),
@@ -2174,45 +2213,48 @@ case_opt_data(E, Cs0) ->
 	    {ok,Es,Cs}
     catch
 	throw:impossible ->
+	    %% The pattern contained a binary or map.
 	    {error,Cs0}
     end.
 
-case_opt_data_1([{[P|Ps0],C,PsAcc,Bs0}|Cs], Es, TypeSig) ->
-    {ok,Ps1,Bs1} = case_data_pat(P, TypeSig),
-    [{Ps1++Ps0,C,PsAcc,Bs1++Bs0}|
-     case_opt_data_1(Cs, Es, TypeSig)];
+case_opt_data_1([{[P0|Ps0],C,PsAcc,Bs0}|Cs], Es, TypeSig) ->
+    P = case_opt_compiler_generated(P0),
+    BindTo = #c_var{name=dummy},
+    {Ps1,[{BindTo,_}|Bs1]} = case_data_pat_alias(P, BindTo, TypeSig, []),
+    [{Ps1++Ps0,C,PsAcc,Bs1++Bs0}|case_opt_data_1(Cs, Es, TypeSig)];
 case_opt_data_1([], _, _) -> [].
 
-%% case_data_pat(Pattern, Type, Arity) -> {ok,[Pattern],[{AliasVar,Pat}]} | error.
-
-case_data_pat(P, TypeSig) ->
-    case cerl:is_data(P) of
-	false ->
-	    case_data_pat_var(P, TypeSig);
-	true ->
-	    {ok,cerl:data_es(P),[]}
-    end.
-
-%% case_data_pat_var(Pattern, {DataType,ArityType}) ->
-%%          {ok,[Pattern],[{AliasVar,Pat}]}
-
-case_data_pat_var(P, {Type,Arity}=TypeSig) ->
-    %% If the entire case statement is evaluated in an effect
-    %% context (e.g. "case {A,B} of ... end, ok"), there will
-    %% be a warning that a term is constructed but never used.
-    %% To avoid that warning, we must annotate the data
-    %% constructor as compiler generated.
-    Ann = [compiler_generated|cerl:get_ann(P)],
+case_data_pat_alias(P, BindTo0, TypeSig, Bs0) ->
     case cerl:type(P) of
-	var ->
-	    Vars = make_vars(cerl:get_ann(P), Arity),
-	    {ok,Vars,[{P,cerl:ann_make_data(Ann, Type, Vars)}]};
 	alias ->
-	    V = cerl:alias_var(P),
-	    Apat = cerl:alias_pat(P),
-	    {ok,Ps,Bs} = case_data_pat(Apat, TypeSig),
-	    {ok,Ps,[{V,cerl:ann_make_data(Ann, Type,
-					  pat_to_expr_list(Ps))}|Bs]}
+	    %% Recursively handle the pattern and bind to
+	    %% the alias variable.
+	    BindTo = cerl:alias_var(P),
+	    Apat0 = cerl:alias_pat(P),
+	    Ann = [compiler_generated],
+	    Apat = cerl:set_ann(Apat0, Ann),
+	    {Ps,Bs} = case_data_pat_alias(Apat, BindTo, TypeSig, Bs0),
+	    {Ps,[{BindTo0,BindTo}|Bs]};
+	var ->
+	    %% Here we will need to actually build the data and bind
+	    %% it to the variable.
+	    {Type,Arity} = TypeSig,
+	    Vars = make_vars([], Arity),
+	    Ann = [compiler_generated],
+	    Data = cerl:ann_make_data(Ann, Type, Vars),
+	    Bs = [{BindTo0,P},{P,Data}|Bs0],
+	    {Vars,Bs};
+	_ ->
+	    %% Since case_opt_nomatch/3 has removed all clauses that
+	    %% cannot match, we KNOW that this clause must match and
+	    %% that the pattern must be a data constructor.
+	    %% Here we must build the data and bind it to the variable.
+	    {Type,_} = TypeSig,
+	    DataEs = cerl:data_es(P),
+	    Vars = pat_to_expr_list(DataEs),
+	    Ann = [compiler_generated],
+	    Data = cerl:ann_make_data(Ann, Type, Vars),
+	    {DataEs,[{BindTo0,Data}]}
     end.
 
 %% pat_to_expr(Pattern) -> Expression.
@@ -2259,18 +2301,11 @@ make_var_name() ->
     list_to_atom("fol"++integer_to_list(N)).
 
 letify(Bs, Body) ->
+    Ann = cerl:get_ann(Body),
     foldr(fun({V,Val}, B) ->
-		  letify(V, Val, B)
+		  cerl:ann_c_let(Ann, [V], Val, B)
 	  end, Body, Bs).
 
-letify(#c_var{name=Vname}=Var, Val, Body) ->
-    case core_lib:is_var_used(Vname, Body) of
-	true ->
-	    A = element(2, Body),
-	    #c_let{anno=A,vars=[Var],arg=Val,body=Body};
-	false -> Body
-    end.
-
 %% opt_case_in_let(LetExpr) -> LetExpr'
 
 opt_case_in_let(#c_let{vars=Vs,arg=Arg,body=B}=Let, Sub) ->
@@ -2635,7 +2670,7 @@ opt_simple_let_2(Let0, Vs0, Arg0, Body0, effect, Sub) ->
 		    opt_case_in_let_arg(opt_case_in_let(Let, Sub), effect, Sub)
 	    end
     end;
-opt_simple_let_2(Let, Vs0, Arg0, Body, value, Sub) ->
+opt_simple_let_2(Let0, Vs0, Arg0, Body, value, Sub) ->
     case {Vs0,Arg0,Body} of
 	{[#c_var{name=N1}],Arg,#c_var{name=N2}} ->
 	    case N1 =:= N2 of
@@ -2654,9 +2689,17 @@ opt_simple_let_2(Let, Vs0, Arg0, Body, value, Sub) ->
 	    %% can be evaluated in effect context to simplify it.
 	    expr(#c_seq{arg=Arg,body=Body}, value, sub_new_preserve_types(Sub));
 	{Vs,Arg,Body} ->
-	    opt_case_in_let_arg(
-	      opt_case_in_let(Let#c_let{vars=Vs,arg=Arg,body=Body}, Sub),
-	      value, Sub)
+	    %% If none of the variables are used in the body, we can rewrite the
+	    %% let to a sequence:
+	    %%   let <Var> = Arg in BodyWithoutVar  ==> seq Arg BodyWithoutVar
+	    case is_any_var_used(Vs, Body) of
+		false ->
+		    expr(#c_seq{arg=Arg,body=Body}, value,
+			 sub_new_preserve_types(Sub));
+		true ->
+		    Let = Let0#c_let{vars=Vs,arg=Arg,body=Body},
+		    opt_case_in_let_arg(opt_case_in_let(Let, Sub), value, Sub)
+	    end
     end.
 
 move_case_into_arg(#c_case{arg=#c_let{vars=OuterVars0,arg=OuterArg,
@@ -3106,7 +3149,7 @@ add_bin_opt_info(Core, Term) ->
     end.
 
 add_warning(Core, Term) ->
-    case is_compiler_generated(Core) of
+    case suppress_warning(Core) of
 	true ->
 	    ok;
 	false ->
@@ -3131,9 +3174,17 @@ get_file([{file,File}|_]) -> File;
 get_file([_|T]) -> get_file(T);
 get_file([]) -> "no_file". % should not happen
 
+suppress_warning(Core) ->
+    is_compiler_generated(Core) orelse
+	is_result_unwanted(Core).
+
 is_compiler_generated(Core) ->
-    Anno = core_lib:get_anno(Core),
-    member(compiler_generated, Anno).
+    Ann = cerl:get_ann(Core),
+    member(compiler_generated, Ann).
+
+is_result_unwanted(Core) ->
+    Ann = cerl:get_ann(Core),
+    member(result_not_wanted, Ann).
 
 get_warnings() ->
     ordsets:from_list((erase({?MODULE,warnings}))).
diff --git a/lib/compiler/src/v3_core.erl b/lib/compiler/src/v3_core.erl
index a9b54005d1..f0b90ff31c 100644
--- a/lib/compiler/src/v3_core.erl
+++ b/lib/compiler/src/v3_core.erl
@@ -515,16 +515,16 @@ exprs([], St) -> {[],St}.
 %%  Generate an internal core expression.
 
 expr({var,L,V}, St) -> {#c_var{anno=lineno_anno(L, St),name=V},[],St};
-expr({char,L,C}, St) -> {#c_literal{anno=lineno_anno(L, St),val=C},[],St};
-expr({integer,L,I}, St) -> {#c_literal{anno=lineno_anno(L, St),val=I},[],St};
-expr({float,L,F}, St) -> {#c_literal{anno=lineno_anno(L, St),val=F},[],St};
-expr({atom,L,A}, St) -> {#c_literal{anno=lineno_anno(L, St),val=A},[],St};
-expr({nil,L}, St) -> {#c_literal{anno=lineno_anno(L, St),val=[]},[],St};
-expr({string,L,S}, St) -> {#c_literal{anno=lineno_anno(L, St),val=S},[],St};
+expr({char,L,C}, St) -> {#c_literal{anno=full_anno(L, St),val=C},[],St};
+expr({integer,L,I}, St) -> {#c_literal{anno=full_anno(L, St),val=I},[],St};
+expr({float,L,F}, St) -> {#c_literal{anno=full_anno(L, St),val=F},[],St};
+expr({atom,L,A}, St) -> {#c_literal{anno=full_anno(L, St),val=A},[],St};
+expr({nil,L}, St) -> {#c_literal{anno=full_anno(L, St),val=[]},[],St};
+expr({string,L,S}, St) -> {#c_literal{anno=full_anno(L, St),val=S},[],St};
 expr({cons,L,H0,T0}, St0) ->
     {H1,Hps,St1} = safe(H0, St0),
     {T1,Tps,St2} = safe(T0, St1),
-    A = lineno_anno(L, St2),
+    A = full_anno(L, St2),
     {annotate_cons(A, H1, T1, St2),Hps ++ Tps,St2};
 expr({lc,L,E,Qs0}, St0) ->
     {Qs1,St1} = preprocess_quals(L, Qs0, St0),
@@ -536,7 +536,7 @@ expr({tuple,L,Es0}, St0) ->
     A = record_anno(L, St1),
     {annotate_tuple(A, Es1, St1),Eps,St1};
 expr({map,L,Es0}, St0) ->
-    map_build_pairs(#c_literal{val=#{}}, Es0, lineno_anno(L, St0), St0);
+    map_build_pairs(#c_literal{val=#{}}, Es0, full_anno(L, St0), St0);
 expr({map,L,M0,Es0}, St0) ->
     try expr_map(M0,Es0,lineno_anno(L, St0),St0) of
 	{_,_,_}=Res -> Res
@@ -551,7 +551,7 @@ expr({map,L,M0,Es0}, St0) ->
 		    args=As},[],St}
     end;
 expr({bin,L,Es0}, St0) ->
-    try expr_bin(Es0, lineno_anno(L, St0), St0) of
+    try expr_bin(Es0, full_anno(L, St0), St0) of
 	{_,_,_}=Res -> Res
     catch
 	throw:bad_binary ->
@@ -641,11 +641,11 @@ expr({'catch',L,E0}, St0) ->
     Lanno = lineno_anno(L, St1),
     {#icatch{anno=#a{anno=Lanno},body=Eps ++ [E1]},[],St1};
 expr({'fun',L,{function,F,A},{_,_,_}=Id}, St) ->
-    Lanno = lineno_anno(L, St),
+    Lanno = full_anno(L, St),
     {#c_var{anno=Lanno++[{id,Id}],name={F,A}},[],St};
 expr({'fun',L,{function,M,F,A}}, St0) ->
     {As,Aps,St1} = safe_list([M,F,A], St0),
-    Lanno = lineno_anno(L, St1),
+    Lanno = full_anno(L, St1),
     {#icall{anno=#a{anno=Lanno},
 	    module=#c_literal{val=erlang},
 	    name=#c_literal{val=make_fun},
@@ -656,13 +656,9 @@ expr({named_fun,L,'_',Cs,Id}, St) ->
     fun_tq(Id, Cs, L, St, unnamed);
 expr({named_fun,L,Name,Cs,Id}, St) ->
     fun_tq(Id, Cs, L, St, {named,Name});
-expr({call,L,{remote,_,M,F},As0}, #core{wanted=Wanted}=St0) ->
+expr({call,L,{remote,_,M,F},As0}, St0) ->
     {[M1,F1|As1],Aps,St1} = safe_list([M,F|As0], St0),
-    Lanno = lineno_anno(L, St1),
-    Anno = case Wanted of
-	       false -> [result_not_wanted|Lanno];
-	       true -> Lanno
-	   end,
+    Anno = full_anno(L, St1),
     {#icall{anno=#a{anno=Anno},module=M1,name=F1,args=As1},Aps,St1};
 expr({call,Lc,{atom,Lf,F},As0}, St0) ->
     {As1,Aps,St1} = safe_list(As0, St0),
@@ -724,13 +720,13 @@ expr({op,L,'orelse',E1,E2}, St0) ->
     expr(E, St);
 expr({op,L,Op,A0}, St0) ->
     {A1,Aps,St1} = safe(A0, St0),
-    LineAnno = lineno_anno(L, St1),
+    LineAnno = full_anno(L, St1),
     {#icall{anno=#a{anno=LineAnno},		%Must have an #a{}
 	    module=#c_literal{anno=LineAnno,val=erlang},
 	    name=#c_literal{anno=LineAnno,val=Op},args=[A1]},Aps,St1};
 expr({op,L,Op,L0,R0}, St0) ->
     {As,Aps,St1} = safe_list([L0,R0], St0),
-    LineAnno = lineno_anno(L, St1),
+    LineAnno = full_anno(L, St1),
     {#icall{anno=#a{anno=LineAnno},		%Must have an #a{}
 	    module=#c_literal{anno=LineAnno,val=erlang},
 	    name=#c_literal{anno=LineAnno,val=Op},args=As},Aps,St1}.
@@ -971,7 +967,7 @@ fun_tq({_,_,Name}=Id, Cs0, L, St0, NameInfo) ->
     {Cs1,Ceps,St1} = clauses(Cs0, St0),
     {Args,St2} = new_vars(Arity, St1),
     {Ps,St3} = new_vars(Arity, St2),		%Need new variables here
-    Anno = lineno_anno(L, St3),
+    Anno = full_anno(L, St3),
     Fc = function_clause(Ps, Anno, {Name,Arity}),
     Fun = #ifun{anno=#a{anno=Anno},
 		id=[{id,Id}],				%We KNOW!
@@ -2341,16 +2337,21 @@ record_anno(L, St) when L >= ?REC_OFFSET ->
         true ->
             [record | lineno_anno(L - ?REC_OFFSET, St)];
         false ->
-            lineno_anno(L, St)
+            full_anno(L, St)
     end;
 record_anno(L, St) when L < -?REC_OFFSET ->
     case member(dialyzer, St#core.opts) of
         true ->
             [record | lineno_anno(L + ?REC_OFFSET, St)];
         false ->
-            lineno_anno(L, St)
+            full_anno(L, St)
     end;
 record_anno(L, St) ->
+    full_anno(L, St).
+
+full_anno(L, #core{wanted=false}=St) ->
+    [result_not_wanted|lineno_anno(L, St)];
+full_anno(L, #core{wanted=true}=St) ->
     lineno_anno(L, St).
 
 lineno_anno(L, St) ->
diff --git a/lib/compiler/test/compilation_SUITE.erl b/lib/compiler/test/compilation_SUITE.erl
index 8711f35e8e..296774e083 100644
--- a/lib/compiler/test/compilation_SUITE.erl
+++ b/lib/compiler/test/compilation_SUITE.erl
@@ -611,12 +611,10 @@ otp_7345(Config) when is_list(Config) ->
 
 otp_7345(ObjRef, _RdEnv, Args) ->
     Cid = ObjRef#contextId.cid,
-    _DpRef =
-	#dpRef{cid = Cid,
+    _ =	#dpRef{cid = Cid,
 		     ms_device_context_id = cid_id,
 		     tlli = #ptmsi{value = 0}},
-    _QosProfile =
-	#qosProfileBssgp{peak_bit_rate_msb = 0,
+    _ =	#qosProfileBssgp{peak_bit_rate_msb = 0,
 			 peak_bit_rate_lsb = 80,
 			 t_a_precedence = 49},
     [Cpdu|_] = Args,
diff --git a/lib/compiler/test/core_fold_SUITE.erl b/lib/compiler/test/core_fold_SUITE.erl
index 1358e002e2..40a5ba2b17 100644
--- a/lib/compiler/test/core_fold_SUITE.erl
+++ b/lib/compiler/test/core_fold_SUITE.erl
@@ -23,7 +23,8 @@
 	 t_element/1,setelement/1,t_length/1,append/1,t_apply/1,bifs/1,
 	 eq/1,nested_call_in_case/1,guard_try_catch/1,coverage/1,
 	 unused_multiple_values_error/1,unused_multiple_values/1,
-	 multiple_aliases/1,redundant_boolean_clauses/1,mixed_matching_clauses/1]).
+	 multiple_aliases/1,redundant_boolean_clauses/1,
+	 mixed_matching_clauses/1,unnecessary_building/1]).
 
 -export([foo/0,foo/1,foo/2,foo/3]).
 
@@ -40,7 +41,8 @@ groups() ->
       [t_element,setelement,t_length,append,t_apply,bifs,
        eq,nested_call_in_case,guard_try_catch,coverage,
        unused_multiple_values_error,unused_multiple_values,
-       multiple_aliases,redundant_boolean_clauses,mixed_matching_clauses]}].
+       multiple_aliases,redundant_boolean_clauses,
+       mixed_matching_clauses,unnecessary_building]}].
 
 
 init_per_suite(Config) ->
@@ -252,6 +254,8 @@ do_guard_try_catch(K, V) ->
 	    false
     end.
 
+-record(cover_opt_guard_try, {list=[]}).
+
 coverage(Config) when is_list(Config) ->
     ?line {'EXIT',{{case_clause,{a,b,c}},_}} =
 	(catch cover_will_match_list_type({a,b,c})),
@@ -261,6 +265,9 @@ coverage(Config) when is_list(Config) ->
     ?line error = cover_will_match_lit_list(),
     {ok,[a]} = cover_is_safe_bool_expr(a),
 
+    ok = cover_opt_guard_try(#cover_opt_guard_try{list=[a]}),
+    error = cover_opt_guard_try(#cover_opt_guard_try{list=[]}),
+
     %% Make sure that we don't attempt to make literals
     %% out of pids. (Putting a pid into a #c_literal{}
     %% would crash later compiler passes.)
@@ -314,6 +321,14 @@ cover_is_safe_bool_expr(X) ->
 	    false
     end.
 
+cover_opt_guard_try(Msg) ->
+    if
+	length(Msg#cover_opt_guard_try.list) =/= 1 ->
+	    error;
+	true ->
+	    ok
+    end.
+
 bsm_an_inlined(<<_:8>>, _) -> ok;
 bsm_an_inlined(_, _) -> error.
 
@@ -400,4 +415,29 @@ mixed_matching_clauses(Config) when is_list(Config) ->
       end,
   ok.
 
+unnecessary_building(Config) when is_list(Config) ->
+    Term1 = do_unnecessary_building_1(test_lib:id(a)),
+    [{a,a},{a,a}] = Term1,
+    7 = erts_debug:size(Term1),
+
+    %% The Input term should not be rebuilt (thus, it should
+    %% only be counted once in the size of the combined term).
+    Input = test_lib:id({a,b,c}),
+    Term2 = test_lib:id(do_unnecessary_building_2(Input)),
+    {b,[{a,b,c},none],x} = Term2,
+    4+4+4+2 = erts_debug:size([Term2|Input]),
+
+    ok.
+
+do_unnecessary_building_1(S) ->
+    %% The tuple must only be built once.
+    F0 = F1 = {S,S},
+    [F0,F1].
+
+do_unnecessary_building_2({a,_,_}=T) ->
+    %% The T term should not be rebuilt.
+    {b,
+     [_,_] = [T,none],
+     x}.
+
 id(I) -> I.
diff --git a/lib/compiler/test/test_lib.erl b/lib/compiler/test/test_lib.erl
index e06e42276a..e8f469c5b4 100644
--- a/lib/compiler/test/test_lib.erl
+++ b/lib/compiler/test/test_lib.erl
@@ -20,9 +20,11 @@
 
 -include("test_server.hrl").
 -compile({no_auto_import,[binary_part/2]}).
--export([recompile/1,parallel/0,uniq/0,opt_opts/1,get_data_dir/1,
+-export([id/1,recompile/1,parallel/0,uniq/0,opt_opts/1,get_data_dir/1,
 	 smoke_disasm/1,p_run/2,binary_part/2]).
 
+id(I) -> I.
+
 recompile(Mod) when is_atom(Mod) ->
     case whereis(cover_server) of
  	undefined -> ok;
diff --git a/lib/compiler/test/trycatch_SUITE.erl b/lib/compiler/test/trycatch_SUITE.erl
index b490257a6e..8ab618bb01 100644
--- a/lib/compiler/test/trycatch_SUITE.erl
+++ b/lib/compiler/test/trycatch_SUITE.erl
@@ -790,7 +790,6 @@ nested_after_1({X1,C1,V1},
 
 
 nested_horrid(Config) when is_list(Config) ->
-    _V = {make_ref(),nested_horrid,4.711},
     {[true,true],{[true,1.0],1.0}} =
 	nested_horrid_1({true,void,void}, 1.0),
     ok.
diff --git a/lib/compiler/test/warnings_SUITE.erl b/lib/compiler/test/warnings_SUITE.erl
index 6e5a7d35e9..6663985ad7 100644
--- a/lib/compiler/test/warnings_SUITE.erl
+++ b/lib/compiler/test/warnings_SUITE.erl
@@ -39,7 +39,7 @@
 	 guard/1,bad_arith/1,bool_cases/1,bad_apply/1,
          files/1,effect/1,bin_opt_info/1,bin_construction/1,
 	 comprehensions/1,maps/1,redundant_boolean_clauses/1,
-	 latin1_fallback/1]).
+	 latin1_fallback/1,underscore/1]).
 
 % Default timetrap timeout (set in init_per_testcase).
 -define(default_timeout, ?t:minutes(2)).
@@ -64,7 +64,8 @@ groups() ->
       [pattern,pattern2,pattern3,pattern4,guard,
        bad_arith,bool_cases,bad_apply,files,effect,
        bin_opt_info,bin_construction,comprehensions,maps,
-       redundant_boolean_clauses,latin1_fallback]}].
+       redundant_boolean_clauses,latin1_fallback,
+       underscore]}].
 
 init_per_suite(Config) ->
     Config.
@@ -280,11 +281,12 @@ bad_arith(Config) when is_list(Config) ->
 	     {3,sys_core_fold,{eval_failure,badarith}},
 	     {9,sys_core_fold,nomatch_guard},
 	     {9,sys_core_fold,{eval_failure,badarith}},
+	     {9,sys_core_fold,{no_effect,{erlang,is_integer,1}}},
 	     {10,sys_core_fold,nomatch_guard},
 	     {10,sys_core_fold,{eval_failure,badarith}},
 	     {15,sys_core_fold,{eval_failure,badarith}}
 	    ] }}],
-    ?line [] = run(Config, Ts),
+    [] = run(Config, Ts),
     ok.
 
 bool_cases(Config) when is_list(Config) ->
@@ -678,6 +680,45 @@ latin1_fallback(Conf) when is_list(Conf) ->
 
     ok.
 
+underscore(Config) when is_list(Config) ->
+    S0 = <<"f(A) ->
+              _VAR1 = <<A>>,
+              _VAR2 = {ok,A},
+              _VAR3 = [A],
+              ok.
+	    g(A) ->
+              _VAR1 = A/0,
+              _VAR2 = date(),
+	      ok.
+            h() ->
+               _VAR1 = fun() -> ok end,
+	      ok.
+            i(A) ->
+               _VAR1 = #{A=>42},
+	      ok.
+	 ">>,
+    Ts0 = [{underscore0,
+	    S0,
+	    [],
+	    {warnings,[{2,sys_core_fold,useless_building},
+		       {3,sys_core_fold,useless_building},
+		       {4,sys_core_fold,useless_building},
+		       {7,sys_core_fold,result_ignored},
+		       {8,sys_core_fold,{no_effect,{erlang,date,0}}},
+		       {11,sys_core_fold,useless_building},
+		       {14,sys_core_fold,useless_building}
+		      ]}}],
+    [] = run(Config, Ts0),
+
+    %% Replace all "_VAR<digit>" variables with a plain underscore.
+    %% Now there should be no warnings.
+    S1 = re:replace(S0, "_VAR\\d+", "_", [global]),
+    io:format("~s\n", [S1]),
+    Ts1 = [{underscore1,S1,[],[]}],
+    [] = run(Config, Ts1),
+
+    ok.
+
 %%%
 %%% End of test cases.
 %%%