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/v3_codegen.erl | 50 +++++++++++++++++++++++------------------
 1 file changed, 28 insertions(+), 22 deletions(-)

(limited to 'lib/compiler/src/v3_codegen.erl')

diff --git a/lib/compiler/src/v3_codegen.erl b/lib/compiler/src/v3_codegen.erl
index a96d58a903..a8f4926e55 100644
--- a/lib/compiler/src/v3_codegen.erl
+++ b/lib/compiler/src/v3_codegen.erl
@@ -1495,28 +1495,34 @@ select_extract_map(Src, Vs, Fail, I, Vdb, Bef, St) ->
     {Code, Aft, St}.
 
 
-select_extract_cons(Src, [#k_var{name=Hd}, #k_var{name=Tl}], I, Vdb, Bef, St) ->
-    {Es,Aft} = case {vdb_find(Hd, Vdb), vdb_find(Tl, Vdb)} of
-		   {{_,_,Lhd}, {_,_,Ltl}} when Lhd =< I, Ltl =< I ->
-		       %% Both head and tail are dead.  No need to generate
-		       %% any instruction.
-		       {[], Bef};
-		   _ ->
-		       %% At least one of head and tail will be used,
-		       %% but we must always fetch both.  We will call
-		       %% clear_dead/2 to allow reuse of the register
-		       %% in case only of them is used.
-
-		       Reg0 = put_reg(Tl, put_reg(Hd, Bef#sr.reg)),
-		       Int0 = Bef#sr{reg=Reg0},
-		       Rsrc = fetch_var(Src, Int0),
-		       Rhd = fetch_reg(Hd, Reg0),
-		       Rtl = fetch_reg(Tl, Reg0),
-		       Int1 = clear_dead(Int0, I, Vdb),
-		       {[{get_list,Rsrc,Rhd,Rtl}], Int1}
-	       end,
-    {Es,Aft,St}.
-    
+select_extract_cons(Src, [#k_var{name=Hd},#k_var{name=Tl}], I, Vdb, Bef, St) ->
+    Rsrc = fetch_var(Src, Bef),
+    Int = clear_dead(Bef, I, Vdb),
+    {{_,_,Lhd},{_,_,Ltl}} = {vdb_find(Hd, Vdb),vdb_find(Tl, Vdb)},
+    case {Lhd =< I, Ltl =< I} of
+        {true,true} ->
+            %% Both dead.
+            {[],Bef,St};
+        {true,false} ->
+            %% Head dead.
+            Reg0 = put_reg(Tl, Bef#sr.reg),
+            Aft = Int#sr{reg=Reg0},
+            Rtl = fetch_reg(Tl, Reg0),
+            {[{get_tl,Rsrc,Rtl}],Aft,St};
+        {false,true} ->
+            %% Tail dead.
+            Reg0 = put_reg(Hd, Bef#sr.reg),
+            Aft = Int#sr{reg=Reg0},
+            Rhd = fetch_reg(Hd, Reg0),
+            {[{get_hd,Rsrc,Rhd}],Aft,St};
+        {false,false} ->
+            %% Both used.
+            Reg0 = put_reg(Tl, put_reg(Hd, Bef#sr.reg)),
+            Aft = Bef#sr{reg=Reg0},
+            Rhd = fetch_reg(Hd, Reg0),
+            Rtl = fetch_reg(Tl, Reg0),
+            {[{get_hd,Rsrc,Rhd},{get_tl,Rsrc,Rtl}],Aft,St}
+    end.
 
 guard_clause_cg(#k_guard_clause{anno=#l{vdb=Vdb},guard=G,body=B}, Fail, Bef, St0) ->
     {Gis,Int,St1} = guard_cg(G, Fail, Vdb, Bef, St0),
-- 
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