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.
---
 erts/emulator/beam/instrs.tab | 10 ++++++++++
 erts/emulator/beam/ops.tab    |  3 +++
 2 files changed, 13 insertions(+)

(limited to 'erts/emulator')

diff --git a/erts/emulator/beam/instrs.tab b/erts/emulator/beam/instrs.tab
index f19027b1ec..6a531fcc09 100644
--- a/erts/emulator/beam/instrs.tab
+++ b/erts/emulator/beam/instrs.tab
@@ -322,6 +322,16 @@ get_list(Src, Hd, Tl) {
     $Tl = tl;
 }
 
+get_hd(Src, Hd) {
+    Eterm* tmp_ptr = list_val($Src);
+    $Hd = CAR(tmp_ptr);
+}
+
+get_tl(Src, Tl) {
+    Eterm* tmp_ptr = list_val($Src);
+    $Tl = CDR(tmp_ptr);
+}
+
 i_get(Src, Dst) {
     $Dst = erts_pd_hash_get(c_p, $Src);
 }
diff --git a/erts/emulator/beam/ops.tab b/erts/emulator/beam/ops.tab
index 1f4a8eadb0..77e375f2c0 100644
--- a/erts/emulator/beam/ops.tab
+++ b/erts/emulator/beam/ops.tab
@@ -182,6 +182,9 @@ get_list r x y
 get_list r y r
 get_list r x r
 
+get_hd xy xy
+get_tl xy xy
+
 # Old-style catch.
 catch y f
 catch_end y
-- 
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