Index: lib/MC/MCParser/AsmLexer.cpp
===================================================================
--- lib/MC/MCParser/AsmLexer.cpp
+++ lib/MC/MCParser/AsmLexer.cpp
@@ -459,8 +459,11 @@
 }
 
 bool AsmLexer::isAtStartOfComment(char Char) {
-  // FIXME: This won't work for multi-character comment indicators like "//".
-  return Char == *MAI.getCommentString();
+  const char *CommentString = MAI.getCommentString();
+  // handle "//" specially since it is the only real two character CommentString
+  if (CommentString[1] == '/')
+    return Char == CommentString[0] && *CurPtr == CommentString[1];
+  return Char == *CommentString;
 }
 
 bool AsmLexer::isAtStatementSeparator(const char *Ptr) {
Index: test/MC/AArch64/single-slash.s
===================================================================
--- /dev/null
+++ test/MC/AArch64/single-slash.s
@@ -0,0 +1,6 @@
+// RUN: llvm-mc -triple aarch64-none-linux-gnu < %s | FileCheck %s
+
+// Test that a single slash is not mistaken as the start of comment.
+
+//CHECK: movz    x0, #0x10
+    mov x0, #(32 / 2)