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.

1 files changed, 5 insertions, 2 deletions

diff --git a/lib/compiler/src/beam_utils.erl b/lib/compiler/src/beam_utils.erl
index 5333925589..4dcce30583 100644
--- a/lib/compiler/src/beam_utils.erl
+++ b/lib/compiler/src/beam_utils.erl
@@ -602,8 +602,11 @@ check_liveness(R, [{test_heap,N,Live}|Is], St) ->
 check_liveness(R, [{allocate_zero,N,Live}|Is], St) ->
     I = {block,[{set,[],[],{alloc,Live,{zero,N,0,[]}}}]},
     check_liveness(R, [I|Is], St);
-check_liveness(R, [{get_list,S,D1,D2}|Is], St) ->
-    I = {block,[{set,[D1,D2],[S],get_list}]},
+check_liveness(R, [{get_hd,S,D}|Is], St) ->
+    I = {block,[{set,[D],[S],get_hd}]},
+    check_liveness(R, [I|Is], St);
+check_liveness(R, [{get_tl,S,D}|Is], St) ->
+    I = {block,[{set,[D],[S],get_tl}]},
     check_liveness(R, [I|Is], St);
 check_liveness(R, [remove_message|Is], St) ->
     check_liveness(R, Is, St);