Fix slow compilation of huge functions

Some huge functions would compile very slowly because of a bottleneck
in `beam_ssa:def_used/2`. One example is the `cuter_binlib` module in
https://github.com/cuter-testing/cuter. On my computer, this commit
reduces the compilatation time for `cuter_binlib` to 45 seconds down
from more than 4 minutes.

Noticed-by: Kostis Sagonas

1 files changed, 13 insertions, 7 deletions

diff --git a/lib/compiler/src/beam_ssa.erl b/lib/compiler/src/beam_ssa.erl
index afd38dcd08..6492d1e1bf 100644
--- a/lib/compiler/src/beam_ssa.erl
+++ b/lib/compiler/src/beam_ssa.erl
@@ -123,7 +123,7 @@
                       'copy' | 'put_tuple_arity' | 'put_tuple_element' |
                       'put_tuple_elements' | 'set_tuple_element'.
 
--import(lists, [foldl/3,keyfind/3,mapfoldl/3,member/2,reverse/1]).
+-import(lists, [foldl/3,keyfind/3,mapfoldl/3,member/2,reverse/1,umerge/1]).
 
 -spec add_anno(Key, Value, Construct) -> Construct when
       Key :: atom(),
@@ -651,12 +651,18 @@ is_commutative('=/=') -> true;
 is_commutative('/=') -> true;
 is_commutative(_) -> false.
 
-def_used_1([#b_blk{is=Is,last=Last}|Bs], Preds, Def0, Used0) ->
-    {Def,Used1} = def_used_is(Is, Preds, Def0, Used0),
-    Used = ordsets:union(used(Last), Used1),
-    def_used_1(Bs, Preds, Def, Used);
-def_used_1([], _Preds, Def, Used) ->
-    {ordsets:from_list(Def),Used}.
+def_used_1([#b_blk{is=Is,last=Last}|Bs], Preds, Def0, UsedAcc) ->
+    {Def,Used} = def_used_is(Is, Preds, Def0, used(Last)),
+    case Used of
+        [] ->
+            def_used_1(Bs, Preds, Def, UsedAcc);
+        [_|_] ->
+            def_used_1(Bs, Preds, Def, [Used|UsedAcc])
+    end;
+def_used_1([], _Preds, Def0, UsedAcc) ->
+    Def = ordsets:from_list(Def0),
+    Used = umerge(UsedAcc),
+    {Def,Used}.
 
 def_used_is([#b_set{op=phi,dst=Dst,args=Args}|Is],
             Preds, Def0, Used0) ->