diff --git a/llvm/include/llvm/MC/MCAsmInfo.h b/llvm/include/llvm/MC/MCAsmInfo.h
--- a/llvm/include/llvm/MC/MCAsmInfo.h
+++ b/llvm/include/llvm/MC/MCAsmInfo.h
@@ -130,6 +130,14 @@
   /// at the beginning of statements. Defaults to false.
   bool RestrictCommentStringToStartOfStatement = false;
 
+  /// This indicates whether to allow additional "comment strings" to be lexed
+  /// as a comment. Setting this attribute to true, will ensure that C-style
+  /// line comments (// ..), C-style block comments (/* .. */), and "#" are
+  /// all treated as comments in addition to the string specified by the
+  /// CommentString attribute.
+  /// Default is true.
+  bool AllowAdditionalComments = true;
+
   /// This is appended to emitted labels.  Defaults to ":"
   const char *LabelSuffix;
 
@@ -567,6 +575,7 @@
   bool getRestrictCommentStringToStartOfStatement() const {
     return RestrictCommentStringToStartOfStatement;
   }
+  bool shouldAllowAdditionalComments() const { return AllowAdditionalComments; }
   const char *getLabelSuffix() const { return LabelSuffix; }
 
   bool useAssignmentForEHBegin() const { return UseAssignmentForEHBegin; }
diff --git a/llvm/lib/MC/MCParser/AsmLexer.cpp b/llvm/lib/MC/MCParser/AsmLexer.cpp
--- a/llvm/lib/MC/MCParser/AsmLexer.cpp
+++ b/llvm/lib/MC/MCParser/AsmLexer.cpp
@@ -175,7 +175,13 @@
 
 /// LexSlash: Slash: /
 ///           C-Style Comment: /* ... */
+///           C-style Comment: // ...
 AsmToken AsmLexer::LexSlash() {
+  if (!MAI.shouldAllowAdditionalComments()) {
+    IsAtStartOfStatement = false;
+    return AsmToken(AsmToken::Slash, StringRef(TokStart, 1));
+  }
+
   switch (*CurPtr) {
   case '*':
     IsAtStartOfStatement = false;
@@ -729,7 +735,9 @@
       UnLex(TokenBuf[0]);
       return AsmToken(AsmToken::HashDirective, s);
     }
-    return LexLineComment();
+
+    if (MAI.shouldAllowAdditionalComments())
+      return LexLineComment();
   }
 
   if (isAtStartOfComment(TokStart))
diff --git a/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmInfo.cpp b/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmInfo.cpp
--- a/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmInfo.cpp
+++ b/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmInfo.cpp
@@ -23,6 +23,7 @@
 
   CommentString = AssemblerDialect == AD_HLASM ? "*" : "#";
   RestrictCommentStringToStartOfStatement = (AssemblerDialect == AD_HLASM);
+  AllowAdditionalComments = (AssemblerDialect == AD_ATT);
   ZeroDirective = "\t.space\t";
   Data64bitsDirective = "\t.quad\t";
   UsesELFSectionDirectiveForBSS = true;
diff --git a/llvm/unittests/MC/SystemZ/SystemZAsmLexerTest.cpp b/llvm/unittests/MC/SystemZ/SystemZAsmLexerTest.cpp
--- a/llvm/unittests/MC/SystemZ/SystemZAsmLexerTest.cpp
+++ b/llvm/unittests/MC/SystemZ/SystemZAsmLexerTest.cpp
@@ -32,6 +32,9 @@
     RestrictCommentStringToStartOfStatement = Value;
   }
   void setCommentString(StringRef Value) { CommentString = Value; }
+  void setAllowAdditionalComments(bool Value) {
+    AllowAdditionalComments = Value;
+  }
 };
 
 // Setup a testing class that the GTest framework can call.
@@ -213,4 +216,155 @@
        AsmToken::EndOfStatement, AsmToken::Eof});
   lexAndCheckTokens(AsmStr, ExpectedTokens);
 }
+
+TEST_F(SystemZAsmLexerTest, DontCheckStrictCommentString) {
+  StringRef AsmStr = "# abc\n/* def *///  xyz";
+
+  // Setup.
+  setupCallToAsmParser(AsmStr);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  SmallVector<AsmToken::TokenKind> ExpectedTokens(
+      {AsmToken::EndOfStatement, AsmToken::Comment, AsmToken::EndOfStatement,
+       AsmToken::Eof});
+  lexAndCheckTokens(AsmStr, ExpectedTokens);
+}
+
+TEST_F(SystemZAsmLexerTest, DontCheckStrictCommentString2) {
+  StringRef AsmStr = "# abc\n/* def *///  xyz\n* rst";
+
+  // Setup.
+  MUPMAI->setCommentString("*");
+  setupCallToAsmParser(AsmStr);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  SmallVector<AsmToken::TokenKind> ExpectedTokens(
+      {AsmToken::EndOfStatement, AsmToken::Comment, AsmToken::EndOfStatement,
+       AsmToken::EndOfStatement, AsmToken::Eof});
+  lexAndCheckTokens(AsmStr, ExpectedTokens);
+}
+
+TEST_F(SystemZAsmLexerTest, CheckStrictCommentString) {
+  StringRef AsmStr = "# abc\n/* def *///  xyz";
+
+  // Setup.
+  MUPMAI->setAllowAdditionalComments(false);
+  setupCallToAsmParser(AsmStr);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  // "# abc" -> still treated as a comment, since CommentString
+  //            is set to "#"
+  SmallVector<AsmToken::TokenKind> ExpectedTokens;
+  ExpectedTokens.push_back(AsmToken::EndOfStatement); // "# abc\n"
+  ExpectedTokens.push_back(AsmToken::Slash);          // "/"
+  ExpectedTokens.push_back(AsmToken::Star);           // "*"
+  ExpectedTokens.push_back(AsmToken::Identifier);     // "def"
+  ExpectedTokens.push_back(AsmToken::Star);           // "*"
+  ExpectedTokens.push_back(AsmToken::Slash);          // "/"
+  ExpectedTokens.push_back(AsmToken::Slash);          // "/"
+  ExpectedTokens.push_back(AsmToken::Slash);          // "/"
+  ExpectedTokens.push_back(AsmToken::Identifier);     // "xyz"
+  ExpectedTokens.push_back(AsmToken::EndOfStatement);
+  ExpectedTokens.push_back(AsmToken::Eof);
+
+  lexAndCheckTokens(AsmStr, ExpectedTokens);
+}
+
+TEST_F(SystemZAsmLexerTest, CheckStrictCommentString2) {
+  StringRef AsmStr = "// abc";
+
+  // Setup.
+  MUPMAI->setAllowAdditionalComments(false);
+  MUPMAI->setCommentString("//");
+  setupCallToAsmParser(AsmStr);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  // "// abc" -> will still be treated as a comment because "//" is the
+  //             CommentString
+  SmallVector<AsmToken::TokenKind> ExpectedTokens(
+      {AsmToken::EndOfStatement, AsmToken::Eof});
+  lexAndCheckTokens(AsmStr /* "// abc" */, ExpectedTokens);
+}
+
+TEST_F(SystemZAsmLexerTest, CheckStrictCommentString3) {
+  StringRef AsmStr = "/* abc */";
+
+  // Setup.
+  MUPMAI->setAllowAdditionalComments(false);
+  setupCallToAsmParser(AsmStr);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  SmallVector<AsmToken::TokenKind> ExpectedTokens;
+  ExpectedTokens.push_back(AsmToken::Slash);
+  ExpectedTokens.push_back(AsmToken::Star);
+  ExpectedTokens.push_back(AsmToken::Identifier);
+  ExpectedTokens.push_back(AsmToken::Star);
+  ExpectedTokens.push_back(AsmToken::Slash);
+  ExpectedTokens.push_back(AsmToken::EndOfStatement);
+  ExpectedTokens.push_back(AsmToken::Eof);
+
+  lexAndCheckTokens(AsmStr, ExpectedTokens);
+}
+
+TEST_F(SystemZAsmLexerTest, CheckStrictCommentString4) {
+  StringRef AsmStr = "# abc\n/* def *///  xyz";
+
+  // Setup.
+  MUPMAI->setCommentString("*");
+  MUPMAI->setAllowAdditionalComments(false);
+  MUPMAI->setRestrictCommentStringToStartOfStatement(true);
+  setupCallToAsmParser(AsmStr);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  SmallVector<AsmToken::TokenKind> ExpectedTokens;
+  ExpectedTokens.push_back(AsmToken::Hash);           // "#"
+  ExpectedTokens.push_back(AsmToken::Identifier);     // "abc"
+  ExpectedTokens.push_back(AsmToken::EndOfStatement); // "\n"
+  ExpectedTokens.push_back(AsmToken::Slash);          // "/"
+  ExpectedTokens.push_back(AsmToken::Star);           // "*"
+  ExpectedTokens.push_back(AsmToken::Identifier);     // "def"
+  ExpectedTokens.push_back(AsmToken::Star);           // "*"
+  ExpectedTokens.push_back(AsmToken::Slash);          // "/"
+  ExpectedTokens.push_back(AsmToken::Slash);          // "/"
+  ExpectedTokens.push_back(AsmToken::Slash);          // "/"
+  ExpectedTokens.push_back(AsmToken::Identifier);     // "xyz"
+  ExpectedTokens.push_back(AsmToken::EndOfStatement);
+  ExpectedTokens.push_back(AsmToken::Eof);
+
+  lexAndCheckTokens(AsmStr, ExpectedTokens);
+}
+
+TEST_F(SystemZAsmLexerTest, CheckStrictCommentString5) {
+  StringRef AsmStr = "#abc\n/* def */// xyz";
+
+  // Setup.
+  MUPMAI->setCommentString("*");
+  MUPMAI->setAllowAdditionalComments(false);
+  setupCallToAsmParser(AsmStr);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  SmallVector<AsmToken::TokenKind> ExpectedTokens;
+  ExpectedTokens.push_back(AsmToken::Hash);           // "#"
+  ExpectedTokens.push_back(AsmToken::Identifier);     // "abc"
+  ExpectedTokens.push_back(AsmToken::EndOfStatement); // "\n"
+  ExpectedTokens.push_back(AsmToken::Slash);          // "/"
+  ExpectedTokens.push_back(AsmToken::EndOfStatement); // "* def */// xyz"
+  ExpectedTokens.push_back(AsmToken::Eof);
+
+  lexAndCheckTokens(AsmStr, ExpectedTokens);
+}
 } // end anonymous namespace