From 00c13ed24a08799ba938a64e3198e75f07329486 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Tue, 29 Aug 2017 18:02:58 +0200
Subject: Eliminate unnecessary 'move' instructions

The compiler could sometimes emit unnecessary 'move'
instructions in the code for binary matching, for
example for this function:

escape(<<Byte, Rest/bits>>, Pos) when Byte >= 127 ->
    escape(Rest, Pos + 1);
escape(<<Byte, Rest/bits>>, Pos) ->
    escape(Rest, Pos + Byte);
escape(<<_Rest/bits>>, Pos) ->
    Pos.

The generated code would look like this:

{function, escape, 2, 2}.
  {label,1}.
    {line,[{location,"t.erl",17}]}.
    {func_info,{atom,t},{atom,escape},2}.
  {label,2}.
    {test,bs_start_match2,{f,1},2,[{x,0},0],{x,0}}.
    {test,bs_get_integer2,
          {f,4},
          2,
          [{x,0},
           {integer,8},
           1,
           {field_flags,[{anno,[17,{file,"t.erl"}]},unsigned,big]}],
          {x,2}}.
    {'%',{bin_opt,[17,{file,"t.erl"}]}}.
    {move,{x,0},{x,3}}.                              %% UNECESSARY!
    {test,is_ge,{f,3},[{x,2},{integer,127}]}.
    {line,[{location,"t.erl",18}]}.
    {gc_bif,'+',{f,0},4,[{x,1},{integer,1}],{x,1}}.
    {move,{x,3},{x,0}}.                              %% UNECESSARY!
    {call_only,2,{f,2}}.
  {label,3}.
    {line,[{location,"t.erl",20}]}.
    {gc_bif,'+',{f,0},4,[{x,1},{x,2}],{x,1}}.
    {move,{x,3},{x,0}}.                              %% UNECESSARY!
    {call_only,2,{f,2}}.
  {label,4}.
    {move,{x,1},{x,0}}.
    return.

The redundant 'move' instructions have been marked.

To avoid the 'move' instructions, we can extend the existing
function is_context_unused/1 in v3_codegen. If v3_codegen can
know that the match context will not be used again, it can reuse
the register for the match context and avoid the extra 'move'
instructions.

https://bugs.erlang.org/browse/ERL-444
---
 lib/compiler/src/v3_codegen.erl | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

(limited to 'lib/compiler/src')

diff --git a/lib/compiler/src/v3_codegen.erl b/lib/compiler/src/v3_codegen.erl
index 47c1567f10..0ae3289103 100644
--- a/lib/compiler/src/v3_codegen.erl
+++ b/lib/compiler/src/v3_codegen.erl
@@ -884,12 +884,19 @@ select_extract_bin([{var,Hd}], Size, Unit, binary, Flags, Vf,
 %%   calculcated by v3_life is too conservative to be useful for this purpose.)
 %%   'true' means that the code that follows will definitely not use the context
 %%   again (because it is a block, not guard or matching code); 'false' that we
-%%   are not sure (there is either a guard, or more matching, either which may
-%%   reference the context again).
-
-is_context_unused(#l{ke=Ke}) -> is_context_unused(Ke);
-is_context_unused({block,_}) -> true;
-is_context_unused(_) -> false.
+%%   are not sure (there could be more matching).
+
+is_context_unused(#l{ke=Ke}) ->
+    is_context_unused(Ke);
+is_context_unused({alt,_First,Then}) ->
+    %% {alt,First,Then} can be used for different purposes. If the Then part
+    %% is a block, it means that matching has finished and is used for a guard
+    %% to choose between the matched clauses.
+    is_context_unused(Then);
+is_context_unused({block,_}) ->
+    true;
+is_context_unused(_) ->
+    false.
 
 select_bin_end(#l{ke={val_clause,{bin_end,Ctx},B}},
 	       Ivar, Tf, Bef, St0) ->
-- 
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