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, 6 insertions, 0 deletions

diff --git a/lib/compiler/src/beam_validator.erl b/lib/compiler/src/beam_validator.erl
index f8bf935132..7e5d86c177 100644
--- a/lib/compiler/src/beam_validator.erl
+++ b/lib/compiler/src/beam_validator.erl
@@ -591,6 +591,12 @@ valfun_4({get_list,Src,D1,D2}, Vst0) ->
     assert_type(cons, Src, Vst0),
     Vst = set_type_reg(term, D1, Vst0),
     set_type_reg(term, D2, Vst);
+valfun_4({get_hd,Src,Dst}, Vst) ->
+    assert_type(cons, Src, Vst),
+    set_type_reg(term, Dst, Vst);
+valfun_4({get_tl,Src,Dst}, Vst) ->
+    assert_type(cons, Src, Vst),
+    set_type_reg(term, Dst, Vst);
 valfun_4({get_tuple_element,Src,I,Dst}, Vst) ->
     assert_type({tuple_element,I+1}, Src, Vst),
     set_type_reg(term, Dst, Vst);