Fix compiler crash in AsmParser::Lex

[alexandru.guduleasa]

Summary

When an empty comment is present in an assembly file, the compiler will crash because it checks the first character for '\n' or '\r'.
The fix consists of also checking if the string is empty before accessing the *front* method of the StringRef.
A test is included for the x86 target, but this issue is reproducible with other targets as well.

[niravd]

"AsmParser::parseStatement" also has this issue. Please add the fix there as part of your commit as well?

Modulo that, LGTM.

lib/MC/MCParser/AsmParser.cpp

703 ↗

(On Diff #101721)

Nit: Please use !getTok().getString().empty()

[alexandru.guduleasa]

Updated as per comments.

[niravd]

Looks good. Do you have commit access or would you like someone to commit this patch for you?

[alexandru.guduleasa]

Thank you for the review.
I do not have commit access; I would appreciate it if someone can commit this.

llvm/trunk/lib/MC/MCParser/AsmParser.cpp

 
 const AsmToken &AsmParser::Lex() {
   if (Lexer.getTok().is(AsmToken::Error))
     Error(Lexer.getErrLoc(), Lexer.getErr());
 
   // if it's a end of statement with a comment in it
   if (getTok().is(AsmToken::EndOfStatement)) {
     // if this is a line comment output it.
-    if (getTok().getString().front() != '\n' &&
+    if (!getTok().getString().empty() && getTok().getString().front() != '\n' &&
         getTok().getString().front() != '\r' && MAI.preserveAsmComments())
       Out.addExplicitComment(Twine(getTok().getString()));
   }
 
   const AsmToken *tok = &Lexer.Lex();
 
   // Parse comments here to be deferred until end of next statement.
   while (tok->is(AsmToken::Comment)) {

 bool AsmParser::parseStatement(ParseStatementInfo &Info,
                                MCAsmParserSemaCallback *SI) {
   assert(!hasPendingError() && "parseStatement started with pending error");
   // Eat initial spaces and comments
   while (Lexer.is(AsmToken::Space))
     Lex();
   if (Lexer.is(AsmToken::EndOfStatement)) {
     // if this is a line comment we can drop it safely
-    if (getTok().getString().front() == '\r' ||
+    if (getTok().getString().empty() || getTok().getString().front() == '\r' ||
         getTok().getString().front() == '\n')
       Out.AddBlankLine();
     Lex();
     return false;
   }
   if (Lexer.is(AsmToken::Hash)) {
     // Seeing a hash here means that it was an end-of-line comment in
     // an asm syntax where hash's are not comment and the previous

llvm/trunk/test/MC/AsmParser/empty-comment.s

+  #RUN: llvm-mc -preserve-comments -n -triple i386-linux-gnu < %s > %t
+  .text
+foo:
+  nop #
 No newline at end of file