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, 8 insertions, 0 deletions

diff --git a/lib/compiler/src/beam_a.erl b/lib/compiler/src/beam_a.erl
index 7df2edd714..91acb19971 100644
--- a/lib/compiler/src/beam_a.erl
+++ b/lib/compiler/src/beam_a.erl
@@ -61,6 +61,14 @@ rename_instrs([{'%live',_}|Is]) ->
     %% Ignore old type of live annotation. Only happens when compiling
     %% from very old .S files.
     rename_instrs(Is);
+rename_instrs([{get_list,S,D1,D2}|Is]) ->
+    %% Only happens when compiling from old .S files.
+    if
+        D1 =:= S ->
+            [{get_tl,S,D2},{get_hd,S,D1}|rename_instrs(Is)];
+        true ->
+            [{get_hd,S,D1},{get_tl,S,D2}|rename_instrs(Is)]
+    end;
 rename_instrs([I|Is]) ->
     [rename_instr(I)|rename_instrs(Is)];
 rename_instrs([]) -> [].