Slightly optimize updating of maps

The instruction put_map_assoc/5 (used for updating a map) has a
failure operand, but it can't actually fail provided that its "map"
argument is a map.

The following code:

    M#{key=>value}.

will be compiled to:

    {test,is_map,{f,3},[{x,0}]}.
    {line,[...]}.
    {put_map_assoc,{f,0},{x,0},{x,0},1,{list,[{atom,key},{atom,value}]}}.
    return.
  {label,3}.
    %% Code that produces a 'badmap' exception follows.

Because of the is_map instruction, {x,0} always contains a map when
the put_map_assoc instruction is executed. Therefore we can remove
the failure operand. That will save one word, and also eliminate
two tests at run-time.

The only problem is that the compiler in OTP 17 did not emit a
is_map instruction before the put_map_assoc instruction. Therefore,
we must add an instruction that tests for a map if the code was
compiled with the OTP 17 compiler.

Unfortunately, there is no safe and relatively easy way to known that
the OTP 17 compiler was used, so we will check whether a compiler
before OTP 20 was used. OTP 20 introduced a new chunk type for atoms,
which is trivial to check.

1 files changed, 39 insertions, 10 deletions

diff --git a/erts/emulator/beam/ops.tab b/erts/emulator/beam/ops.tab
index fdc4506351..92e67bb470 100644
--- a/erts/emulator/beam/ops.tab
+++ b/erts/emulator/beam/ops.tab
@@ -1299,23 +1299,52 @@ apply I
 apply_last I P
 
 #
-# Map instructions in R17.
+# Handle compatibility with OTP 17 here.
 #
 
-sorted_put_map_assoc/5
-put_map_assoc F Map Dst Live Size Rest=* | map_key_sort(Size, Rest) => \
-  sorted_put_map_assoc F Map Dst Live Size Rest
+i_put_map_assoc/4
+
+# We KNOW that in OTP 20 (actually OTP 18 and higher), a put_map_assoc instruction
+# is always preceded by an is_map test. That means that put_map_assoc can never
+# fail and does not need any failure label.
+
+put_map_assoc Fail Map Dst Live Size Rest=* | compiled_with_otp_20_or_higher() => \
+	      i_put_map_assoc Map Dst Live Size Rest
+
+# Translate the put_map_assoc instruction if the module was compiled by a compiler
+# before 20. This is only necessary if the OTP 17 compiler was used, but we
+# have no safe and relatively easy way to know whether OTP 18/19 was used.
+
+put_map_assoc Fail=p Map Dst Live Size Rest=* => \
+	      ensure_map Map | i_put_map_assoc Map Dst Live Size Rest
+put_map_assoc Fail=f Map Dst Live Size Rest=* => \
+	      is_map Fail Map | i_put_map_assoc Map Dst Live Size Rest
+
+ensure_map Lit=q | literal_is_map(Lit) =>
+ensure_map Src=cqy => move Src x | ensure_map x
+
+%cold
+ensure_map x
+%hot
+
+#
+# Map instructions. First introduced in R17.
+#
+
+sorted_put_map_assoc/4
+i_put_map_assoc Map Dst Live Size Rest=* | map_key_sort(Size, Rest) => \
+  sorted_put_map_assoc Map Dst Live Size Rest
 
 sorted_put_map_exact/5
 put_map_exact F Map Dst Live Size Rest=* | map_key_sort(Size, Rest) => \
   sorted_put_map_exact F Map Dst Live Size Rest
 
-sorted_put_map_assoc j Map Dst Live Size Rest=* | is_empty_map(Map) => \
+sorted_put_map_assoc Map Dst Live Size Rest=* | is_empty_map(Map) => \
    new_map Dst Live Size Rest
-sorted_put_map_assoc F Src=s Dst Live Size Rest=* => \
-	       update_map_assoc F Src Dst Live Size Rest
-sorted_put_map_assoc F Src Dst Live Size Rest=* => \
-	       move Src x | update_map_assoc F x Dst Live Size Rest
+sorted_put_map_assoc Src=s Dst Live Size Rest=* => \
+	       update_map_assoc Src Dst Live Size Rest
+sorted_put_map_assoc Src Dst Live Size Rest=* => \
+	       move Src x | update_map_assoc x Dst Live Size Rest
 
 sorted_put_map_exact F Src=s Dst Live Size Rest=* => \
 	      update_map_exact F Src Dst Live Size Rest
@@ -1327,7 +1356,7 @@ new_map Dst Live Size Rest=* | is_small_map_literal_keys(Size, Rest) => \
 
 new_map d I I
 i_new_small_map_lit d I q
-update_map_assoc j s d I I
+update_map_assoc s d I I
 update_map_exact j s d I I
 
 is_map Fail Lit=q | literal_is_map(Lit) =>