beam_bsm: Optimize btb_index()

lists:dropwhile/2 and the fun in btb_index_1/2 shows up in the
top 10 list of eprof. Replace dropwhile with a special-purpose
function for a tiny increase in speed.

lib/compiler/src/beam_bsm.erl

@@ -20,7 +20,7 @@
 -module(beam_bsm).
 -export([module/2,format_error/1]).
 
--import(lists, [member/2,foldl/3,reverse/1,sort/1,all/2,dropwhile/2]).
+-import(lists, [member/2,foldl/3,reverse/1,sort/1,all/2]).
 
 %%%
 %%% We optimize bit syntax matching where the tail end of a binary is
@@ -542,16 +542,13 @@ btb_context_regs_1(Regs, N, Tag, Acc) ->
 %%  a binary. MustSave is true if the function may pass the match
 %%  context to the bs_context_to_binary instruction (in which case
 %%  the current position in the binary must have saved into the
-%%  start position using "bs_save_2 Ctx start".
+%%  start position using "bs_save_2 Ctx start").
 
 btb_index(Fs) ->
     btb_index_1(Fs, []).
 
 btb_index_1([{function,_,_,Entry,Is0}|Fs], Acc0) ->
-    [{label,Entry}|Is] =
-	dropwhile(fun({label,L}) when L =:= Entry -> false;
-		     (_) -> true
-		  end, Is0),
+    Is = drop_to_label(Is0, Entry),
     Acc = btb_index_2(Is, Entry, false, Acc0),
     btb_index_1(Fs, Acc);
 btb_index_1([], Acc) -> gb_trees:from_orddict(sort(Acc)).
@@ -566,6 +563,9 @@ btb_index_2(Is0, Entry, _, Acc) ->
 	throw:none -> Acc
     end.
 
+drop_to_label([{label,L}|Is], L) -> Is;
+drop_to_label([_|Is], L) -> drop_to_label(Is, L).
+
 btb_index_find_start_match([{test,_,{f,F},_},{bs_context_to_binary,_}|Is]) ->
     btb_index_find_label(Is, F);
 btb_index_find_start_match(_) ->