Compile external fun expressions to literals

The expressions fun M:F/A, when all elements are literals are also
treated as a literal. Since they have consistent representation and
don't depend on the code currently loaded in the VM, this is safe.
This can provide significant performance improvements in code using such
functions extensively - a full function call to erlang:make_fun/3 is
replaced by a single move instruction and no register shuffling or
saving registers to stack is necessary. Additionally, compound data
types that contain such external functions as elements can be treated as
literals too.

The commit also changes the representation of external funs to be a
valid Erlang syntax and adds support for literal external funs to core
Erlang.

6 files changed, 20 insertions, 3 deletions

diff --git a/lib/compiler/src/cerl.erl b/lib/compiler/src/cerl.erl
index 6b936a7687..fce23bfd68 100644
--- a/lib/compiler/src/cerl.erl
+++ b/lib/compiler/src/cerl.erl
@@ -433,6 +433,8 @@ is_literal_term(T) when is_tuple(T) ->
 is_literal_term(B) when is_bitstring(B) -> true;
 is_literal_term(M) when is_map(M) ->
     is_literal_term_list(maps:to_list(M));
+is_literal_term(F) when is_function(F) ->
+    erlang:fun_info(F, type) =:= {type,external};
 is_literal_term(_) ->
     false.
 
diff --git a/lib/compiler/src/core_parse.yrl b/lib/compiler/src/core_parse.yrl
index 79a7cccd98..f828b3ed56 100644
--- a/lib/compiler/src/core_parse.yrl
+++ b/lib/compiler/src/core_parse.yrl
@@ -36,7 +36,7 @@ other_pattern atomic_pattern tuple_pattern cons_pattern tail_pattern
 binary_pattern segment_patterns segment_pattern
 
 expression single_expression
-literal literals atomic_literal tuple_literal cons_literal tail_literal
+literal literals atomic_literal tuple_literal cons_literal tail_literal fun_literal
 nil tuple cons tail
 binary segments segment
 
@@ -267,6 +267,7 @@ single_expression -> cons : '$1'.
 single_expression -> binary : '$1'.
 single_expression -> variable : '$1'.
 single_expression -> function_name : '$1'.
+single_expression -> fun_literal : '$1'.
 single_expression -> fun_expr : '$1'.
 single_expression -> let_expr : '$1'.
 single_expression -> letrec_expr : '$1'.
@@ -303,6 +304,9 @@ tail_literal -> ']' : #c_literal{val=[]}.
 tail_literal -> '|' literal ']' : '$2'.
 tail_literal -> ',' literal tail_literal : c_cons('$2', '$3').
 
+fun_literal -> 'fun' atom ':' atom '/' integer :
+	#c_literal{val = erlang:make_fun(tok_val('$2'), tok_val('$4'), tok_val('$6'))}.
+
 tuple -> '{' '}' : c_tuple([]).
 tuple -> '{' anno_expressions '}' : c_tuple('$2').
 
diff --git a/lib/compiler/src/core_pp.erl b/lib/compiler/src/core_pp.erl
index 2516a9a1e1..f247722b4c 100644
--- a/lib/compiler/src/core_pp.erl
+++ b/lib/compiler/src/core_pp.erl
@@ -136,6 +136,11 @@ format_1(#c_literal{anno=A,val=M},Ctxt) when is_map(M) ->
 			  key=#c_literal{val=K},
 			  val=#c_literal{val=V}} || {K,V} <- Pairs],
     format_1(#c_map{anno=A,arg=#c_literal{val=#{}},es=Cpairs},Ctxt);
+format_1(#c_literal{val=F},_Ctxt) when is_function(F) ->
+    {module,M} = erlang:fun_info(F, module),
+    {name,N} = erlang:fun_info(F, name),
+    {arity,A} = erlang:fun_info(F, arity),
+    ["fun ",core_atom(M),$:,core_atom(N),$/,integer_to_list(A)];
 format_1(#c_var{name={I,A}}, _) ->
     [core_atom(I),$/,integer_to_list(A)];
 format_1(#c_var{name=V}, _) ->
@@ -541,4 +546,3 @@ segs_from_bitstring(Bitstring) ->
 	      unit=#c_literal{val=1},
 	      type=#c_literal{val=integer},
 	      flags=#c_literal{val=[unsigned,big]}}].
-
diff --git a/lib/compiler/src/erl_bifs.erl b/lib/compiler/src/erl_bifs.erl
index bafa9d75b7..8fab2400f7 100644
--- a/lib/compiler/src/erl_bifs.erl
+++ b/lib/compiler/src/erl_bifs.erl
@@ -109,6 +109,7 @@ is_pure(erlang, list_to_integer, 1) -> true;
 is_pure(erlang, list_to_pid, 1) -> true;
 is_pure(erlang, list_to_tuple, 1) -> true;
 is_pure(erlang, max, 2) -> true;
+is_pure(erlang, make_fun, 3) -> true;
 is_pure(erlang, min, 2) -> true;
 is_pure(erlang, phash, 2) -> false;
 is_pure(erlang, pid_to_list, 1) -> true;
@@ -196,6 +197,7 @@ is_safe(erlang, is_port, 1) -> true;
 is_safe(erlang, is_reference, 1) -> true;
 is_safe(erlang, is_tuple, 1) -> true;
 is_safe(erlang, make_ref, 0) -> true;
+is_safe(erlang, make_fun, 3) -> true;
 is_safe(erlang, max, 2) -> true;
 is_safe(erlang, min, 2) -> true;
 is_safe(erlang, node, 0) -> true;
diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl
index a9bd363ee1..e39b9df218 100644
--- a/lib/compiler/src/sys_core_fold.erl
+++ b/lib/compiler/src/sys_core_fold.erl
@@ -392,7 +392,7 @@ expr(#c_receive{clauses=Cs0,timeout=T0,action=A0}=Recv, Ctxt, Sub) ->
 expr(#c_apply{anno=Anno,op=Op0,args=As0}=App, _, Sub) ->
     Op1 = expr(Op0, value, Sub),
     As1 = expr_list(As0, value, Sub),
-    case cerl:is_data(Op1) of
+    case cerl:is_data(Op1) andalso not is_literal_fun(Op1) of
         false ->
 	    App#c_apply{op=Op1,args=As1};
 	true ->
@@ -487,6 +487,9 @@ bitstr_list(Es, Sub) ->
 bitstr(#c_bitstr{val=Val,size=Size}=BinSeg, Sub) ->
     BinSeg#c_bitstr{val=expr(Val, Sub),size=expr(Size, value, Sub)}.
 
+is_literal_fun(#c_literal{val=F}) -> is_function(F);
+is_literal_fun(_) -> false.
+
 %% is_safe_simple(Expr, Sub) -> true | false.
 %%  A safe simple cannot fail with badarg and is safe to use
 %%  in a guard.
diff --git a/lib/stdlib/src/erl_parse.yrl b/lib/stdlib/src/erl_parse.yrl
index 14ca24362e..0c338b5952 100644
--- a/lib/stdlib/src/erl_parse.yrl
+++ b/lib/stdlib/src/erl_parse.yrl
@@ -1377,6 +1377,8 @@ normalise({map,_,Pairs}=M) ->
 		({map_field_assoc,_,K,V}) -> {normalise(K),normalise(V)};
 		(_) -> erlang:error({badarg,M})
 	    end, Pairs));
+normalise({'fun',_,{function,{atom,_,M},{atom,_,F},{integer,_,A}}}) ->
+    fun M:F/A;
 %% Special case for unary +/-.
 normalise({op,_,'+',{char,_,I}}) -> I;
 normalise({op,_,'+',{integer,_,I}}) -> I;