From fbcff5a137e37edd80aca9c5fe18ce6880648169 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Thu, 25 Jan 2018 10:09:59 +0100
Subject: Eliminate get_list/3 internally in the compiler

Instructions that produce more than one result complicate
optimizations. get_list/3 is one of two instructions that
produce multiple results (get_map_elements/3 is the other).

Introduce the get_hd/2 and get_tl/2 instructions
that return the head and tail of a cons cell, respectively,
and use it internally in all optimization passes.

For efficiency, we still want to use get_list/3 if both
head and tail are used, so we will translate matching pairs
of get_hd and get_tl back to get_list instructions.
---
 lib/compiler/src/beam_z.erl | 30 +++++++++++++++++++++++++-----
 1 file changed, 25 insertions(+), 5 deletions(-)

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

diff --git a/lib/compiler/src/beam_z.erl b/lib/compiler/src/beam_z.erl
index 1c56b95a9e..6c3a6995d7 100644
--- a/lib/compiler/src/beam_z.erl
+++ b/lib/compiler/src/beam_z.erl
@@ -24,18 +24,20 @@
 
 -export([module/2]).
 
--import(lists, [dropwhile/2]).
+-import(lists, [dropwhile/2,map/2]).
 
 -spec module(beam_utils:module_code(), [compile:option()]) ->
                     {'ok',beam_asm:module_code()}.
 
-module({Mod,Exp,Attr,Fs0,Lc}, _Opt) ->
-    Fs = [function(F) || F <- Fs0],
+module({Mod,Exp,Attr,Fs0,Lc}, Opts) ->
+    NoGetHdTl = proplists:get_bool(no_get_hd_tl, Opts),
+    Fs = [function(F, NoGetHdTl) || F <- Fs0],
     {ok,{Mod,Exp,Attr,Fs,Lc}}.
 
-function({function,Name,Arity,CLabel,Is0}) ->
+function({function,Name,Arity,CLabel,Is0}, NoGetHdTl) ->
     try
-	Is = undo_renames(Is0),
+	Is1 = undo_renames(Is0),
+        Is = maybe_eliminate_get_hd_tl(Is1, NoGetHdTl),
 	{function,Name,Arity,CLabel,Is}
     catch
         Class:Error:Stack ->
@@ -65,6 +67,10 @@ undo_renames([{bif,raise,_,_,_}=I|Is0]) ->
 		      (_) -> true
 		   end, Is0),
     [I|undo_renames(Is)];
+undo_renames([{get_hd,Src,Dst1},{get_tl,Src,Dst2}|Is]) ->
+    [{get_list,Src,Dst1,Dst2}|undo_renames(Is)];
+undo_renames([{get_tl,Src,Dst2},{get_hd,Src,Dst1}|Is]) ->
+    [{get_list,Src,Dst1,Dst2}|undo_renames(Is)];
 undo_renames([I|Is]) ->
     [undo_rename(I)|undo_renames(Is)];
 undo_renames([]) -> [].
@@ -107,3 +113,17 @@ undo_rename({get_map_elements,Fail,Src,{list,List}}) ->
 undo_rename({select,I,Reg,Fail,List}) ->
     {I,Reg,Fail,{list,List}};
 undo_rename(I) -> I.
+
+%%%
+%%% Eliminate get_hd/get_tl instructions if requested by
+%%% the no_get_hd_tl option.
+%%%
+
+maybe_eliminate_get_hd_tl(Is, true) ->
+    map(fun({get_hd,Cons,Hd}) ->
+                {get_list,Cons,Hd,{x,1022}};
+           ({get_tl,Cons,Tl}) ->
+                {get_list,Cons,{x,1022},Tl};
+           (I) -> I
+        end, Is);
+maybe_eliminate_get_hd_tl(Is, false) -> Is.
-- 
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