diff --git a/crypto/perlasm/arm-xlate.pl b/crypto/perlasm/arm-xlate.pl
index eb8ea59b0..655be61bf 100755
--- a/crypto/perlasm/arm-xlate.pl
+++ b/crypto/perlasm/arm-xlate.pl
@@ -168,6 +168,15 @@ while(my $line=<>) {
     $line =~ s|^\s+||;		# ... and skip white spaces in beginning...
     $line =~ s|\s+$||;		# ... and at the end
 
+    if ($flavour =~ /64/) {
+	my $copy = $line;
+	# Also remove line comments.
+	$copy =~ s|//.*||;
+	if ($copy =~ /\b[wx]18\b/) {
+	    die "r18 is reserved by the platform and may not be used.";
+	}
+    }
+
     {
 	$line =~ s|[\b\.]L(\w{2,})|L$1|g;	# common denominator for Locallabel
 	$line =~ s|\bL(\w{2,})|\.L$1|g	if ($dotinlocallabels);
diff --git a/crypto/test/abi_test.h b/crypto/test/abi_test.h
index 44547f812..a3c4bb872 100644
--- a/crypto/test/abi_test.h
+++ b/crypto/test/abi_test.h
@@ -149,11 +149,11 @@ struct alignas(16) Reg128 {
 // AAPCS64: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055b/IHI0055B_aapcs64.pdf
 // iOS64: https://developer.apple.com/library/archive/documentation/Xcode/Conceptual/iPhoneOSABIReference/Articles/ARM64FunctionCallingConventions.html
 //
-// In aarch64, r19 (x19 in a 64-bit context) is the platform register. iOS says
-// user code may not touch it. We found no clear reference for Linux. The iOS
-// behavior implies portable assembly cannot use it, and aarch64 has many
-// registers. Thus this framework ignores register's existence. We can test r19
-// violations with grep.
+// In aarch64, r18 (accessed as w18 or x18 in a 64-bit context) is the platform
+// register. iOS says user code may not touch it. We found no clear reference
+// for Linux. The iOS behavior implies portable assembly cannot use it, and
+// aarch64 has many registers. Thus this framework ignores register's existence.
+// We test r18 violations in arm-xlate.pl.
 #define LOOP_CALLER_STATE_REGISTERS()                                \
   /* Per AAPCS64, section 5.1.2, only the bottom 64 bits of v8-v15 */ \
   /* are preserved. These are accessed as dN. */                     \