Introduce a put_tuple2 instruction

Sometimes when building a tuple, there is no way to avoid an
extra `move` instruction. Consider this code:

    make_tuple(A) -> {ok,A}.

The corresponding BEAM code looks like this:

    {test_heap,3,1}.
    {put_tuple,2,{x,1}}.
    {put,{atom,ok}}.
    {put,{x,0}}.
    {move,{x,1},{x,0}}.
    return.

To avoid overwriting the source register `{x,0}`, a `move`
instruction is necessary.

The problem doesn't exist when building a list:

    %% build_list(A) -> [A].
    {test_heap,2,1}.
    {put_list,{x,0},nil,{x,0}}.
    return.

Introduce a new `put_tuple2` instruction that builds a tuple in a
single instruction, so that the `move` instruction can be eliminated:

    %% make_tuple(A) -> {ok,A}.
    {test_heap,3,1}.
    {put_tuple2,{x,0},{list,[{atom,ok},{x,0}]}}.
    return.

Note that the BEAM loader already combines `put_tuple` and `put`
instructions into an internal instruction similar to `put_tuple2`.
Therefore the introduction of the new instruction will not speed up
execution of tuple building itself, but it will be less work for
the loader to load the new instruction.

1 files changed, 10 insertions, 7 deletions

diff --git a/erts/emulator/beam/instrs.tab b/erts/emulator/beam/instrs.tab
index 42c1168f85..da1dd3dc45 100644
--- a/erts/emulator/beam/instrs.tab
+++ b/erts/emulator/beam/instrs.tab
@@ -559,17 +559,19 @@ update_list(Hd, Dst) {
     HTOP += 2;
 }
 
-i_put_tuple := i_put_tuple.make.fill;
-
-i_put_tuple.make(Dst) {
-    $Dst = make_tuple(HTOP);
-}
-
-i_put_tuple.fill(Arity) {
+put_tuple2(Dst, Arity) {
     Eterm* hp = HTOP;
     Eterm arity = $Arity;
 
+    /*
+     * If operands are not packed (in the 32-bit VM),
+     * is is not safe to use $Dst directly after I
+     * has been updated.
+     */
+    Eterm* dst_ptr = &($Dst);
+
     //| -no_next
+    ASSERT(arity != 0);
     *hp++ = make_arityval(arity);
     I = $NEXT_INSTRUCTION;
     do {
@@ -586,6 +588,7 @@ i_put_tuple.fill(Arity) {
             break;
         }
     } while (--arity != 0);
+    *dst_ptr = make_tuple(HTOP);
     HTOP = hp;
     ASSERT(VALID_INSTR(* (Eterm *)I));
     Goto(*I);