From 47a2ff39ac75c69b41f90f05a63fd9a2e6c0b36a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Sat, 1 Sep 2018 10:57:33 +0200
Subject: 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.
---
 lib/compiler/src/beam_validator.erl | 6 ++++++
 1 file changed, 6 insertions(+)

(limited to 'lib/compiler/src/beam_validator.erl')

diff --git a/lib/compiler/src/beam_validator.erl b/lib/compiler/src/beam_validator.erl
index b44771d8a9..953aced66e 100644
--- a/lib/compiler/src/beam_validator.erl
+++ b/lib/compiler/src/beam_validator.erl
@@ -349,6 +349,12 @@ valfun_1({put_list,A,B,Dst}, Vst0) ->
     assert_term(B, Vst0),
     Vst = eat_heap(2, Vst0),
     set_type_reg(cons, Dst, Vst);
+valfun_1({put_tuple2,Dst,{list,Elements}}, Vst0) ->
+    _ = [assert_term(El, Vst0) || El <- Elements],
+    Size = length(Elements),
+    Vst = eat_heap(Size+1, Vst0),
+    Type = {tuple,Size},
+    set_type_reg(Type, Dst, Vst);
 valfun_1({put_tuple,Sz,Dst}, Vst0) when is_integer(Sz) ->
     Vst1 = eat_heap(1, Vst0),
     Vst = set_type_reg(tuple_in_progress, Dst, Vst1),
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