From 696f47b64c8ad5da71cbffd8fecf0f3bae1e7466 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Thu, 26 Oct 2017 12:57:24 +0200
Subject: Correct erlang:is_builtin/3 for apply/2 and yield/0

erlang:is_builtin(erlang, M, F) returns false for apply/2 and
yield/0. The documentation for erlang:is_builtin/3 says that it
returns true for BIFs that are implemented in C. apply/2 and
yield/0 are implemented in C (as BEAM instructions), and
therefore the correct return value is true.

Also see a similar argument that was made for apply/3 in the past:

    http://erlang.org/pipermail/erlang-bugs/2015-October/005101.html

https://bugs.erlang.org/browse/ERL-500
---
 erts/emulator/beam/beam_emu.c    | 17 ++++++++++-------
 erts/emulator/test/bif_SUITE.erl | 18 ++++++++++++------
 2 files changed, 22 insertions(+), 13 deletions(-)

(limited to 'erts')

diff --git a/erts/emulator/beam/beam_emu.c b/erts/emulator/beam/beam_emu.c
index aa94fbf536..60d0008d8f 100644
--- a/erts/emulator/beam/beam_emu.c
+++ b/erts/emulator/beam/beam_emu.c
@@ -3199,13 +3199,16 @@ erts_is_builtin(Eterm Mod, Eterm Name, int arity)
     Export e;
     Export* ep;
 
-    if (Mod == am_erlang && Name == am_apply && arity == 3) {
-	/*
-	 * Special case. apply/3 is built-in (implemented in C),
-	 * but implemented in a different way than all other
-	 * BIFs.
-	 */
-	return 1;
+    if (Mod == am_erlang) {
+        /*
+         * Special case for built-in functions that are implemented
+         * as instructions as opposed to SNIFs.
+         */
+        if (Name == am_apply && (arity == 2 || arity == 3)) {
+            return 1;
+        } else if (Name == am_yield && arity == 0) {
+            return 1;
+        }
     }
 
     e.info.mfa.module = Mod;
diff --git a/erts/emulator/test/bif_SUITE.erl b/erts/emulator/test/bif_SUITE.erl
index 146c37b118..e1b42e5d85 100644
--- a/erts/emulator/test/bif_SUITE.erl
+++ b/erts/emulator/test/bif_SUITE.erl
@@ -781,14 +781,20 @@ is_builtin(_Config) ->
 		       {F,A} <- M:module_info(exports)],
     Exp = ordsets:from_list(Exp0),
 
-    %% erlang:apply/3 is considered to be built-in, but is not
-    %% implemented as other BIFs.
+    %% Built-ins implemented as special instructions.
+    Instructions = [{erlang,apply,2},{erlang,apply,3},{erlang,yield,0}],
 
-    Builtins0 = [{erlang,apply,3}|erlang:system_info(snifs)],
+    Builtins0 = Instructions ++ erlang:system_info(snifs),
     Builtins = ordsets:from_list(Builtins0),
-    NotBuiltin = ordsets:subtract(Exp, Builtins),
-    _ = [true = erlang:is_builtin(M, F, A) || {M,F,A} <- Builtins],
-    _ = [false = erlang:is_builtin(M, F, A) || {M,F,A} <- NotBuiltin],
+
+    Fakes = [{M,F,42} || {M,F,_} <- Instructions],
+    All = ordsets:from_list(Fakes ++ Exp),
+    NotBuiltin = ordsets:subtract(All, Builtins),
+
+    _ = [{true,_} = {erlang:is_builtin(M, F, A),MFA} ||
+            {M,F,A}=MFA <- Builtins],
+    _ = [{false,_} = {erlang:is_builtin(M, F, A),MFA} ||
+            {M,F,A}=MFA <- NotBuiltin],
 
     ok.
 
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