[AsmParser][ms][X86] Fix possible misbehaviour in parsing of special tokens at start of string.

- Previously, https://reviews.llvm.org/D72680 introduced a new attribute called `AllowSymbolAtNameStart` (in relation to the MAsmParser changes) in `MCAsmInfo.h` which (according to the comment in the header) allows the following behaviour:

```
  /// This is true if the assembler allows $ @ ? characters at the start of
  /// symbol names. Defaults to false.
```

- However, the usage of this field in AsmLexer.cpp doesn't seem completely accurate* for a couple of reasons.

```
  default:
    if (MAI.doesAllowSymbolAtNameStart()) {
      // Handle Microsoft-style identifier: [a-zA-Z_$.@?][a-zA-Z0-9_$.@#?]*
      if (!isDigit(CurChar) &&
          isIdentifierChar(CurChar, MAI.doesAllowAtInName(),
                           AllowHashInIdentifier))
        return LexIdentifier();
    }
```

1. The Dollar and At tokens, when occurring at the start of the string, are treated as separate tokens (AsmToken::Dollar and AsmToken::At respectively) and not lexed as an Identifier.
2. I'm not too sure why `MAI.doesAllowAtInName()` is used when `AllowAtInIdentifier` could be used. For X86 platforms, afaict, this shouldn't be an issue, since the `CommentString` attribute isn't "@". (alternatively the call to the setter can be set anywhere else as needed). The `AllowAtInName` does have an additional important meaning, but in the context of AsmLexer, shouldn't mean anything different compared to `AllowAtInIdentifier`

My proposal is the following:

- Introduce 3 new fields called `AllowQuestionTokenAtStartOfString`, `AllowDollarTokenAtStartOfString` and `AllowAtTokenAtStartOfString` in MCAsmInfo.h which will encapsulate the previously documented behaviour of "allowing $, @, ? characters at the start of symbol names")
- Introduce these fields where "$", "@" are lexed, and treat them as identifiers depending on whether `Allow[Dollar|At]TokenAtStartOfString` is set.
- For the sole case of "?", append it to the existing logic for treating a "default" token as an Identifier.

z/OS (HLASM) will also make use of some of these fields in follow up patches.

completely accurate* - This was based on the comments and the intended behaviour the code. I might have completely misinterpreted it, and if that is the case my sincere apologies. We can close this patch if necessary, if there are no changes to be made :)

Depends on https://reviews.llvm.org/D99374

Reviewed By: Jonathan.Crowther

Differential Revision: https://reviews.llvm.org/D99889

llvm/include/llvm/MC/MCAsmInfo.h

@@ -181,9 +181,26 @@ protected:
   /// Defaults to false.
   bool AllowAtInName = false;
 
-  /// This is true if the assembler allows $ @ ? characters at the start of
-  /// symbol names. Defaults to false.
-  bool AllowSymbolAtNameStart = false;
+  /// This is true if the assembler allows the "?" character at the start of
+  /// of a string to be lexed as an AsmToken::Identifier.
+  /// If the CommentString is also set to "?", setting this option will have
+  /// no effect, and the string will be lexed as a comment.
+  /// Defaults to false.
+  bool AllowQuestionAtStartOfIdentifier = false;
+
+  /// This is true if the assembler allows the "$" character at the start of
+  /// of a string to be lexed as an AsmToken::Identifier.
+  /// If the CommentString is also set to "$", setting this option will have
+  /// no effect, and the string will be lexed as a comment.
+  /// Defaults to false.
+  bool AllowDollarAtStartOfIdentifier = false;
+
+  /// This is true if the assembler allows the "@" character at the start of
+  /// a string to be lexed as an AsmToken::Identifier.
+  /// If the CommentString is also set to "@", setting this option will have
+  /// no effect, and the string will be lexed as a comment.
+  /// Defaults to false.
+  bool AllowAtAtStartOfIdentifier = false;
 
   /// If this is true, symbol names with invalid characters will be printed in
   /// quotes.
@@ -600,7 +617,15 @@ public:
   const char *getCode64Directive() const { return Code64Directive; }
   unsigned getAssemblerDialect() const { return AssemblerDialect; }
   bool doesAllowAtInName() const { return AllowAtInName; }
-  bool doesAllowSymbolAtNameStart() const { return AllowSymbolAtNameStart; }
+  bool doesAllowQuestionAtStartOfIdentifier() const {
+    return AllowQuestionAtStartOfIdentifier;
+  }
+  bool doesAllowAtAtStartOfIdentifier() const {
+    return AllowAtAtStartOfIdentifier;
+  }
+  bool doesAllowDollarAtStartOfIdentifier() const {
+    return AllowDollarAtStartOfIdentifier;
+  }
   bool supportsNameQuoting() const { return SupportsQuotedNames; }
 
   bool doesSupportDataRegionDirectives() const {

llvm/lib/MC/MCParser/AsmLexer.cpp

@@ -144,7 +144,7 @@ AsmToken AsmLexer::LexHexFloatLiteral(bool NoIntDigits) {
   return AsmToken(AsmToken::Real, StringRef(TokStart, CurPtr - TokStart));
 }
 
-/// LexIdentifier: [a-zA-Z_.][a-zA-Z0-9_$.@#?]*
+/// LexIdentifier: [a-zA-Z_$.@?][a-zA-Z0-9_$.@#?]*
 static bool isIdentifierChar(char C, bool AllowAt, bool AllowHash) {
   return isAlnum(C) || C == '_' || C == '$' || C == '.' || C == '?' ||
          (AllowAt && C == '@') || (AllowHash && C == '#');
@@ -769,17 +769,10 @@ AsmToken AsmLexer::LexToken() {
   IsAtStartOfStatement = false;
   switch (CurChar) {
   default:
-    if (MAI.doesAllowSymbolAtNameStart()) {
-      // Handle Microsoft-style identifier: [a-zA-Z_$.@?][a-zA-Z0-9_$.@#?]*
-      if (!isDigit(CurChar) &&
-          isIdentifierChar(CurChar, MAI.doesAllowAtInName(),
-                           AllowHashInIdentifier))
-        return LexIdentifier();
-    } else {
-      // Handle identifier: [a-zA-Z_.][a-zA-Z0-9_$.@]*
-      if (isalpha(CurChar) || CurChar == '_' || CurChar == '.')
-        return LexIdentifier();
-    }
+    // Handle identifier: [a-zA-Z_.?][a-zA-Z0-9_$.@#?]*
+    if (isalpha(CurChar) || CurChar == '_' || CurChar == '.' ||
+        (MAI.doesAllowQuestionAtStartOfIdentifier() && CurChar == '?'))
+      return LexIdentifier();
 
     // Unknown character, emit an error.
     return ReturnError(TokStart, "invalid character in input");
@@ -823,13 +816,18 @@ AsmToken AsmLexer::LexToken() {
   case '}': return AsmToken(AsmToken::RCurly, StringRef(TokStart, 1));
   case '*': return AsmToken(AsmToken::Star, StringRef(TokStart, 1));
   case ',': return AsmToken(AsmToken::Comma, StringRef(TokStart, 1));
-  case '$':
-    if (LexMotorolaIntegers && isHexDigit(*CurPtr)) {
+  case '$': {
+    if (LexMotorolaIntegers && isHexDigit(*CurPtr))
       return LexDigit();
-    }
-
+    if (MAI.doesAllowDollarAtStartOfIdentifier())
+      return LexIdentifier();
     return AsmToken(AsmToken::Dollar, StringRef(TokStart, 1));
-  case '@': return AsmToken(AsmToken::At, StringRef(TokStart, 1));
+  }
+  case '@': {
+    if (MAI.doesAllowAtAtStartOfIdentifier())
+      return LexIdentifier();
+    return AsmToken(AsmToken::At, StringRef(TokStart, 1));
+  }
   case '\\': return AsmToken(AsmToken::BackSlash, StringRef(TokStart, 1));
   case '=':
     if (*CurPtr == '=') {

llvm/lib/Target/X86/MCTargetDesc/X86MCAsmInfo.cpp

@@ -144,7 +144,9 @@ X86MCAsmInfoMicrosoftMASM::X86MCAsmInfoMicrosoftMASM(const Triple &Triple)
   DollarIsPC = true;
   SeparatorString = "\n";
   CommentString = ";";
-  AllowSymbolAtNameStart = true;
+  AllowQuestionAtStartOfIdentifier = true;
+  AllowDollarAtStartOfIdentifier = true;
+  AllowAtAtStartOfIdentifier = true;
 }
 
 void X86MCAsmInfoGNUCOFF::anchor() { }

llvm/unittests/MC/SystemZ/SystemZAsmLexerTest.cpp

@@ -35,6 +35,15 @@ public:
   void setAllowAdditionalComments(bool Value) {
     AllowAdditionalComments = Value;
   }
+  void setAllowQuestionAtStartOfIdentifier(bool Value) {
+    AllowQuestionAtStartOfIdentifier = Value;
+  }
+  void setAllowAtAtStartOfIdentifier(bool Value) {
+    AllowAtAtStartOfIdentifier = Value;
+  }
+  void setAllowDollarAtStartOfIdentifier(bool Value) {
+    AllowDollarAtStartOfIdentifier = Value;
+  }
 };
 
 // Setup a testing class that the GTest framework can call.
@@ -454,4 +463,112 @@ TEST_F(SystemZAsmLexerTest, CheckDefaultFloats) {
   ExpectedTokens.push_back(AsmToken::Eof);
   lexAndCheckTokens(AsmStr, ExpectedTokens);
 }
+
+TEST_F(SystemZAsmLexerTest, CheckDefaultQuestionAtStartOfIdentifier) {
+  StringRef AsmStr = "?lh1?23";
+
+  // Setup.
+  setupCallToAsmParser(AsmStr);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  SmallVector<AsmToken::TokenKind> ExpectedTokens(
+      {AsmToken::Error, AsmToken::Identifier, AsmToken::EndOfStatement,
+       AsmToken::Eof});
+  lexAndCheckTokens(AsmStr, ExpectedTokens);
+}
+
+TEST_F(SystemZAsmLexerTest, CheckAcceptQuestionAtStartOfIdentifier) {
+  StringRef AsmStr = "?????lh1?23";
+
+  // Setup.
+  MUPMAI->setAllowQuestionAtStartOfIdentifier(true);
+  setupCallToAsmParser(AsmStr);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  SmallVector<AsmToken::TokenKind> ExpectedTokens(
+      {AsmToken::Identifier, AsmToken::EndOfStatement, AsmToken::Eof});
+  lexAndCheckTokens(AsmStr, ExpectedTokens);
+}
+
+TEST_F(SystemZAsmLexerTest, CheckDefaultAtAtStartOfIdentifier) {
+  StringRef AsmStr = "@@lh1?23";
+
+  // Setup.
+  MUPMAI->setAllowQuestionAtStartOfIdentifier(true);
+  setupCallToAsmParser(AsmStr);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  SmallVector<AsmToken::TokenKind> ExpectedTokens(
+      {AsmToken::At, AsmToken::At, AsmToken::Identifier,
+       AsmToken::EndOfStatement, AsmToken::Eof});
+  lexAndCheckTokens(AsmStr, ExpectedTokens);
+}
+
+TEST_F(SystemZAsmLexerTest, CheckAcceptAtAtStartOfIdentifier) {
+  StringRef AsmStr = "@@lh1?23";
+
+  // Setup.
+  MUPMAI->setAllowAtAtStartOfIdentifier(true);
+  setupCallToAsmParser(AsmStr);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  SmallVector<AsmToken::TokenKind> ExpectedTokens(
+      {AsmToken::Identifier, AsmToken::EndOfStatement, AsmToken::Eof});
+  lexAndCheckTokens(AsmStr, ExpectedTokens);
+}
+
+TEST_F(SystemZAsmLexerTest, CheckAccpetAtAtStartOfIdentifier2) {
+  StringRef AsmStr = "@@lj1?23";
+
+  // Setup.
+  MUPMAI->setCommentString("@");
+  MUPMAI->setAllowAtAtStartOfIdentifier(true);
+  setupCallToAsmParser(AsmStr);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  // "@@lj1?23" -> still lexed as a comment as that takes precedence.
+  SmallVector<AsmToken::TokenKind> ExpectedTokens(
+      {AsmToken::EndOfStatement, AsmToken::Eof});
+  lexAndCheckTokens(AsmStr, ExpectedTokens);
+}
+
+TEST_F(SystemZAsmLexerTest, CheckDefaultDollarAtStartOfIdentifier) {
+  StringRef AsmStr = "$$ac$c";
+
+  // Setup.
+  setupCallToAsmParser(AsmStr);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  SmallVector<AsmToken::TokenKind> ExpectedTokens(
+      {AsmToken::Dollar, AsmToken::Dollar, AsmToken::Identifier,
+       AsmToken::EndOfStatement, AsmToken::Eof});
+  lexAndCheckTokens(AsmStr, ExpectedTokens);
+}
+
+TEST_F(SystemZAsmLexerTest, CheckAcceptDollarAtStartOfIdentifier) {
+  StringRef AsmStr = "$$ab$c";
+
+  // Setup.
+  MUPMAI->setAllowDollarAtStartOfIdentifier(true);
+  setupCallToAsmParser(AsmStr);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  SmallVector<AsmToken::TokenKind> ExpectedTokens(
+      {AsmToken::Identifier, AsmToken::EndOfStatement, AsmToken::Eof});
+  lexAndCheckTokens(AsmStr, ExpectedTokens);
+}
 } // end anonymous namespace