Teach the loader to pre-compute the hash value for single-key lookups

Let the loader pre-compute the hash value when a single, literal key
is matched as in:

  #{<<"some_key">>:=V} = Map

In my measurements, this optimization resulted in a 30 percent
speedup for short binary keys.

Unfortunately, this optimizization makes no difference for small
maps with less than 32 keys, since the hash value is not used.
Still, there are the following use cases:

* A map used instead of a record with more than 32 entries. I have
seen some applications with huge records.

* Lookup in JSON dictionaries represented as maps.
The hash value will only be used when the map is a hash map
(currently, that means at least 32 entries).

1 files changed, 37 insertions, 5 deletions

diff --git a/erts/emulator/beam/beam_load.c b/erts/emulator/beam/beam_load.c
index 1a6c6c16ad..ee319d0a78 100644
--- a/erts/emulator/beam/beam_load.c
+++ b/erts/emulator/beam/beam_load.c
@@ -4161,9 +4161,30 @@ map_key_sort(LoaderState* stp, GenOpArg Size, GenOpArg* Rest)
     return 1;
 }
 
+static int
+hash_genop_arg(LoaderState* stp, GenOpArg Key, Uint32* hx)
+{
+    switch (Key.type) {
+    case TAG_a:
+	*hx = hashmap_make_hash(Key.val);
+	return 1;
+    case TAG_i:
+	*hx = hashmap_make_hash(make_small(Key.val));
+	return 1;
+    case TAG_n:
+	*hx = hashmap_make_hash(NIL);
+	return 1;
+    case TAG_q:
+	*hx = hashmap_make_hash(stp->literals[Key.val].term);
+	return 1;
+    default:
+	return 0;
+    }
+}
+
 /*
  * Replace a get_map_elements with one key to an instruction with one
- * element
+ * element.
  */
 
 static GenOp*
@@ -4171,18 +4192,29 @@ gen_get_map_element(LoaderState* stp, GenOpArg Fail, GenOpArg Src,
 		    GenOpArg Size, GenOpArg* Rest)
 {
     GenOp* op;
+    GenOpArg Key;
+    Uint32 hx = 0;
 
     ASSERT(Size.type == TAG_u);
 
     NEW_GENOP(stp, op);
     op->next = NULL;
-    op->op = genop_get_map_element_4;
-    op->arity = 4;
-
     op->a[0] = Fail;
     op->a[1] = Src;
     op->a[2] = Rest[0];
-    op->a[3] = Rest[1];
+
+    Key = Rest[0];
+    if (hash_genop_arg(stp, Key, &hx)) {
+	op->arity = 5;
+	op->op = genop_i_get_map_element_hash_5;
+	op->a[3].type = TAG_u;
+	op->a[3].val = (BeamInstr) hx;
+	op->a[4] = Rest[1];
+    } else {
+	op->arity = 4;
+	op->op = genop_i_get_map_element_4;
+	op->a[3] = Rest[1];
+    }
     return op;
 }