beam_emu: Slightly optimize update_map_{assoc,exact}

In the update loop for big maps, the E variable is restored for
each turn of the loop. It only needs to be restored if a garbage
collection has been performed.

Also add a new test case that attempts to force several garbage
collections while updating a map, to help us find bugs with
incorrect restoration of the E variable after a garbage collection.

2 files changed, 59 insertions, 8 deletions

diff --git a/erts/emulator/beam/beam_emu.c b/erts/emulator/beam/beam_emu.c
index f25b9f594d..4e64dce95c 100644
--- a/erts/emulator/beam/beam_emu.c
+++ b/erts/emulator/beam/beam_emu.c
@@ -6572,10 +6572,9 @@ update_map_assoc(Process* p, Eterm* reg, Eterm map, BeamInstr* I)
 		reg[live] = res;
 		erts_garbage_collect(p, 0, reg, live+1);
 		res       = reg[live];
+		E = p->stop;
 	    }
 
-	    E = p->stop;
-
 	    new_p += 2;
 	}
 	return res;
@@ -6781,7 +6780,6 @@ update_map_exact(Process* p, Eterm* reg, Eterm map, BeamInstr* I)
 	res = map;
 	E = p->stop;
 	while(n--) {
-	    /* assoc can't fail */
 	    GET_TERM(new_p[0], new_key);
 	    GET_TERM(new_p[1], val);
 	    hx = hashmap_make_hash(new_key);
@@ -6795,10 +6793,9 @@ update_map_exact(Process* p, Eterm* reg, Eterm map, BeamInstr* I)
 		reg[live] = res;
 		erts_garbage_collect(p, 0, reg, live+1);
 		res       = reg[live];
+		E = p->stop;
 	    }
 
-	    E = p->stop;
-
 	    new_p += 2;
 	}
 	return res;
@@ -6808,7 +6805,7 @@ update_map_exact(Process* p, Eterm* reg, Eterm map, BeamInstr* I)
     num_old = flatmap_get_size(old_mp);
 
     /*
-     * If the old map is empty, create a new map.
+     * If the old map is empty, fail.
      */
 
     if (num_old == 0) {
diff --git a/erts/emulator/test/map_SUITE.erl b/erts/emulator/test/map_SUITE.erl
index 008fa0e11b..72b8ad91ef 100644
--- a/erts/emulator/test/map_SUITE.erl
+++ b/erts/emulator/test/map_SUITE.erl
@@ -78,7 +78,8 @@
 	t_tracing/1,
 
 	%% instruction-level tests
-	t_has_map_fields/1
+	t_has_map_fields/1,
+	y_regs/1
     ]).
 
 -include_lib("stdlib/include/ms_transform.hrl").
@@ -138,7 +139,8 @@ all() -> [
 	t_tracing,
 
 	%% instruction-level tests
-	t_has_map_fields
+	t_has_map_fields,
+	y_regs
     ].
 
 groups() -> [].
@@ -2758,5 +2760,57 @@ has_map_fields_3(#{a:=_,b:=_}) -> true;
 has_map_fields_3(#{[]:=_,42.0:=_}) -> true;
 has_map_fields_3(#{}) -> false.
 
+y_regs(Config) when is_list(Config) ->
+    Val = [length(Config)],
+    Map0 = y_regs_update(#{}, Val),
+    Map2 = y_regs_update(Map0, Val),
+
+    Map3 = maps:from_list([{I,I*I} || I <- lists:seq(1, 100)]),
+    Map4 = y_regs_update(Map3, Val),
+
+    true = is_map(Map2) andalso is_map(Map4),
+
+    ok.
+
+y_regs_update(Map0, Val0) ->
+    Val1 = {t,Val0},
+    K1 = id({key,1}),
+    K2 = id({key,2}),
+    Map1 = Map0#{K1=>K1,
+		 a=>Val0,b=>Val0,c=>Val0,d=>Val0,e=>Val0,
+		 f=>Val0,g=>Val0,h=>Val0,i=>Val0,j=>Val0,
+		 k=>Val0,l=>Val0,m=>Val0,n=>Val0,o=>Val0,
+		 p=>Val0,q=>Val0,r=>Val0,s=>Val0,t=>Val0,
+		 u=>Val0,v=>Val0,w=>Val0,x=>Val0,y=>Val0,
+		 z=>Val0,
+		 aa=>Val0,ab=>Val0,ac=>Val0,ad=>Val0,ae=>Val0,
+		 af=>Val0,ag=>Val0,ah=>Val0,ai=>Val0,aj=>Val0,
+		 ak=>Val0,al=>Val0,am=>Val0,an=>Val0,ao=>Val0,
+		 ap=>Val0,aq=>Val0,ar=>Val0,as=>Val0,at=>Val0,
+		 au=>Val0,av=>Val0,aw=>Val0,ax=>Val0,ay=>Val0,
+		 az=>Val0,
+		 K2=>[a,b,c]},
+    Map2 = Map1#{K1=>K1,
+		 a:=Val1,b:=Val1,c:=Val1,d:=Val1,e:=Val1,
+		 f:=Val1,g:=Val1,h:=Val1,i:=Val1,j:=Val1,
+		 k:=Val1,l:=Val1,m:=Val1,n:=Val1,o:=Val1,
+		 p:=Val1,q:=Val1,r:=Val1,s:=Val1,t:=Val1,
+		 u:=Val1,v:=Val1,w:=Val1,x:=Val1,y:=Val1,
+		 z:=Val1,
+		 aa:=Val1,ab:=Val1,ac:=Val1,ad:=Val1,ae:=Val1,
+		 af:=Val1,ag:=Val1,ah:=Val1,ai:=Val1,aj:=Val1,
+		 ak:=Val1,al:=Val1,am:=Val1,an:=Val1,ao:=Val1,
+		 ap:=Val1,aq:=Val1,ar:=Val1,as:=Val1,at:=Val1,
+		 au:=Val1,av:=Val1,aw:=Val1,ax:=Val1,ay:=Val1,
+		 az:=Val1,
+		 K2=>[a,b,c]},
+
+    %% Traverse the maps to validate them.
+    _ = erlang:phash2({Map1,Map2}, 100000),
+
+    _ = id({K1,K2,Val0,Val1}),			%Force use of Y registers.
+    Map2.
+
+
 %% Use this function to avoid compile-time evaluation of an expression.
 id(I) -> I.