Sort move instructions on the Y register

Sort sequences of `move` instructions on the Y register.

When moving from X registers to Y registers, having the instructions
sorted on Y registers give the loader more opportunities to use
`move_window{3,4,5}` instructions. For examples, the following five
instructions:

    move_xy x(2) y(0)
    move_xy x(1) y(1)
    move_xy x(0) y(2)
    move_xy x(5) y(3)
    move_xy x(4) y(4)

can be replaced with:

    move_window5_xxxxxy x(2) x(1) x(0) x(5) x(4) y(0)

When the Y registers are not ordered so that `move_window5` can be
used, the loader would typically combine the first three moves to a
`move3_xyxyxy` instruction and the last two moves to a
`move2_par_xyxy` instruction.

When moving from Y registers to X registers, sorting on the Y
registers could potentially be more cache-friendly. It could also
be worthwhile investigating a new `move_window` instruction in
the BEAM interpreter that could move values from contiguous Y registers
to X registers.

Note that `scripts/diffable` can generate diffable dissambly files for
the loaded BEAM code:

    $ scripts/diffable --dis 0
    $ scripts/diffable --dis 1
    $ diff -u 0 1

1 files changed, 36 insertions, 2 deletions

diff --git a/lib/compiler/src/beam_block.erl b/lib/compiler/src/beam_block.erl
index 9d8d5b2b0c..707974b2c1 100644
--- a/lib/compiler/src/beam_block.erl
+++ b/lib/compiler/src/beam_block.erl
@@ -22,7 +22,7 @@
 -module(beam_block).
 
 -export([module/2]).
--import(lists, [reverse/1,splitwith/2]).
+-import(lists, [keysort/2,reverse/1,splitwith/2]).
 
 -spec module(beam_utils:module_code(), [compile:option()]) ->
                     {'ok',beam_utils:module_code()}.
@@ -53,7 +53,8 @@ blockify([I|Is0]=IsAll, Acc) ->
     case collect(I) of
 	error -> blockify(Is0, [I|Acc]);
 	Instr when is_tuple(Instr) ->
-	    {Block,Is} = collect_block(IsAll),
+            {Block0,Is} = collect_block(IsAll),
+            Block = sort_moves(Block0),
 	    blockify(Is, [{block,Block}|Acc])
     end;
 blockify([], Acc) -> reverse(Acc).
@@ -117,3 +118,36 @@ embed_lines([{block,B1},{line,_}=Line|T], Acc) ->
 embed_lines([I|Is], Acc) ->
     embed_lines(Is, [I|Acc]);
 embed_lines([], Acc) -> Acc.
+
+%% sort_moves([Instruction]) -> [Instruction].
+%%  Sort move instructions on the Y register to give the loader
+%%  more opportunities for combining instructions.
+
+sort_moves([{set,[{x,_}],[{y,_}],move}=I|Is0]) ->
+    {Moves,Is} = sort_moves_1(Is0, x, y, [I]),
+    Moves ++ sort_moves(Is);
+sort_moves([{set,[{y,_}],[{x,_}],move}=I|Is0]) ->
+    {Moves,Is} = sort_moves_1(Is0, y, x, [I]),
+    Moves ++ sort_moves(Is);
+sort_moves([I|Is]) ->
+    [I|sort_moves(Is)];
+sort_moves([]) -> [].
+
+sort_moves_1([{set,[{x,0}],[_],move}=I|Is], _DTag, _STag, Acc) ->
+    %% The loader sometimes combines a move to x0 with the
+    %% instruction that follows, producing, for example, a move_call
+    %% instruction. Therefore, we don't want include this move
+    %% instruction in the sorting.
+    {sort_on_yreg(Acc)++[I],Is};
+sort_moves_1([{set,[{DTag,_}],[{STag,_}],move}=I|Is], DTag, STag, Acc) ->
+    sort_moves_1(Is, DTag, STag, [I|Acc]);
+sort_moves_1(Is, _DTag, _STag, Acc) ->
+    {sort_on_yreg(Acc),Is}.
+
+sort_on_yreg([{set,[Dst],[Src],move}|_]=Moves) ->
+    case {Dst,Src} of
+        {{y,_},{x,_}} ->
+            keysort(2, Moves);
+        {{x,_},{y,_}} ->
+            keysort(3, Moves)
+    end.