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_ssa_pre_codegen.erl | 23 +++++++++++++----------
 1 file changed, 13 insertions(+), 10 deletions(-)

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

diff --git a/lib/compiler/src/beam_ssa_pre_codegen.erl b/lib/compiler/src/beam_ssa_pre_codegen.erl
index bbc3739eb5..54aa2efaf6 100644
--- a/lib/compiler/src/beam_ssa_pre_codegen.erl
+++ b/lib/compiler/src/beam_ssa_pre_codegen.erl
@@ -79,8 +79,9 @@
                     {'ok',beam_ssa:b_module()}.
 
 module(#b_module{body=Fs0}=Module, Opts) ->
+    FixTuples = proplists:get_bool(no_put_tuple2, Opts),
     ExtraAnnos = proplists:get_bool(dprecg, Opts),
-    Ps = passes(ExtraAnnos),
+    Ps = passes(FixTuples, ExtraAnnos),
     Fs = functions(Fs0, Ps),
     {ok,Module#b_module{body=Fs}}.
 
@@ -113,13 +114,16 @@ functions([], _Ps) -> [].
             }).
 -define(PASS(N), {N,fun N/1}).
 
-passes(ExtraAnnos) ->
+passes(FixTuples, ExtraAnnos) ->
     Ps = [?PASS(assert_no_critical_edges),
 
           %% Preliminaries.
           ?PASS(fix_bs),
           ?PASS(sanitize),
-          ?PASS(fix_tuples),
+          case FixTuples of
+              false -> ignore;
+              true -> ?PASS(fix_tuples)
+          end,
           ?PASS(place_frames),
           ?PASS(fix_receives),
 
@@ -151,10 +155,7 @@ passes(ExtraAnnos) ->
           ?PASS(fix_aliased_regs),
           ?PASS(frame_size),
           ?PASS(turn_yregs)],
-    case ExtraAnnos of
-        false -> [P || P <- Ps, P =/= ignore];
-        true -> Ps
-    end.
+    [P || P <- Ps, P =/= ignore].
 
 function(#b_function{anno=Anno,args=Args,bs=Blocks0,cnt=Count0}=F0, Ps) ->
     try
@@ -693,9 +694,11 @@ prune_phi(#b_set{args=Args0}=Phi, Reachable) ->
 %%%
 
 %% fix_tuples(St0) -> St.
-%%  We must split tuple creation into two instruction to mirror the
-%%  the way tuples are created in BEAM.  Each put_tuple instruction is
-%%  split into put_tuple_arity followed by put_tuple_elements.
+%%  If compatibility with a previous version of Erlang has been
+%%  requested, tuple creation must be split into two instruction to
+%%  mirror the the way tuples are created in BEAM prior to OTP 22.
+%%  Each put_tuple instruction is split into put_tuple_arity followed
+%%  by put_tuple_elements.
 
 fix_tuples(#st{ssa=Blocks0,cnt=Count0}=St) ->
     F = fun (#b_set{op=put_tuple,args=Args}=Put, C0) ->
-- 
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