Move expansion of strings in binaries to v3_core

This speeds up the compilation of binary literals
with string values in them. For example, compiling
a file with a ~340kB binary would yield the following
times by the compiler:

    Compiling "foo"
     parse_module       :   0.130 s    5327.6 kB
     transform_module   :   0.000 s    5327.6 kB
     lint_module        :   0.011 s    5327.8 kB
     expand_module      :   0.508 s   71881.2 kB
     v3_core            :   0.463 s      11.5 kB

Notice the increase in memory and processing time
in expand_module and v3_core. This happened because
expand_module would expand the string in binaries
into chars. For example, the binary <<"foo">>, which
is represented as

    {bin, 1, [
      {bin_element, 1, {string, 1, "foo"}, default, default}
    ]}

would be converted to

    {bin, 1, [
      {bin_element, 1, {char, 1, $f}, default, default},
      {bin_element, 1, {char, 1, $o}, default, default},
      {bin_element, 1, {char, 1, $o}, default, default}
    ]}

However, v3_core would then traverse all of those
characters and convert it into an actual binary, as it
is a literal value.

This patch addresses this issue by moving the expansion
of string into chars to v3_core and only if a literal
value cannot not be built. This reduces the compilation
time of the file mentioned above to the values below:

    Compiling "bar"
     parse_module       :   0.134 s    5327.6 kB
     transform_module   :   0.000 s    5327.6 kB
     lint_module        :   0.005 s    5327.8 kB
     expand_module      :   0.000 s    5328.7 kB
     v3_core            :   0.013 s      11.2 kB

1 files changed, 9 insertions, 5 deletions

diff --git a/lib/stdlib/src/eval_bits.erl b/lib/stdlib/src/eval_bits.erl
index 80667023fb..631faa3be5 100644
--- a/lib/stdlib/src/eval_bits.erl
+++ b/lib/stdlib/src/eval_bits.erl
@@ -67,16 +67,20 @@ expr_grp([Field | FS], Bs0, Lf, Acc) ->
 expr_grp([], Bs0, _Lf, Acc) ->
     {value,Acc,Bs0}.
 
+eval_field({bin_element, _, {string, _, S}, {integer,_,8}, [integer,{unit,1},unsigned,big]}, Bs0, _Fun) ->
+    Latin1 = [C band 16#FF || C <- S],
+    {list_to_binary(Latin1),Bs0};
 eval_field({bin_element, _, {string, _, S}, default, default}, Bs0, _Fun) ->
     Latin1 = [C band 16#FF || C <- S],
     {list_to_binary(Latin1),Bs0};
-eval_field({bin_element, Line, {string, _, S}, Size0, Options0}, Bs, _Fun) ->
-    {_Size,[Type,_Unit,_Sign,Endian]} = 
+eval_field({bin_element, Line, {string, _, S}, Size0, Options0}, Bs0, Fun) ->
+    {Size1,[Type,{unit,Unit},Sign,Endian]} =
         make_bit_type(Line, Size0, Options0),
-    Res = << <<(eval_exp_field1(C, no_size, no_unit,
-				Type, Endian, no_sign))/binary>> ||
+    {value,Size,Bs1} = Fun(Size1, Bs0),
+    Res = << <<(eval_exp_field1(C, Size, Unit,
+				Type, Endian, Sign))/binary>> ||
 	      C <- S >>,
-    {Res,Bs};
+    {Res,Bs1};
 eval_field({bin_element,Line,E,Size0,Options0}, Bs0, Fun) ->
     {value,V,Bs1} = Fun(E, Bs0),
     {Size1,[Type,{unit,Unit},Sign,Endian]} =