Index: llvm/lib/Support/CommandLine.cpp
===================================================================
--- llvm/lib/Support/CommandLine.cpp
+++ llvm/lib/Support/CommandLine.cpp
@@ -688,7 +688,18 @@
          O->getNumOccurrencesFlag() == cl::OneOrMore;
 }
 
-static bool isWhitespace(char C) { return strchr(" \t\n\r\f\v", C); }
+static bool isWhitespace(char C) {
+  switch(C) {
+    case ' ':
+    case '\t':
+    case '\n':
+    case '\r':
+    case '\f':
+    case '\v':
+      return true;
+  }
+  return false;
+}
 
 static bool isQuote(char C) { return C == '\"' || C == '\''; }
 
@@ -798,25 +809,28 @@
   // end of the source string.
   enum { INIT, UNQUOTED, QUOTED } State = INIT;
   for (size_t I = 0, E = Src.size(); I != E; ++I) {
+    // Avoid multiple assert in operator[].
+    char C = Src[I];
+
     // INIT state indicates that the current input index is at the start of
     // the string or between tokens.
     if (State == INIT) {
-      if (isWhitespace(Src[I])) {
+      if (isWhitespace(C)) {
         // Mark the end of lines in response files
-        if (MarkEOLs && Src[I] == '\n')
+        if (MarkEOLs && C == '\n')
           NewArgv.push_back(nullptr);
         continue;
       }
-      if (Src[I] == '"') {
+      if (C == '"') {
         State = QUOTED;
         continue;
       }
-      if (Src[I] == '\\') {
+      if (C == '\\') {
         I = parseBackslash(Src, I, Token);
         State = UNQUOTED;
         continue;
       }
-      Token.push_back(Src[I]);
+      Token.push_back(C);
       State = UNQUOTED;
       continue;
     }
@@ -825,38 +839,38 @@
     // quotes.
     if (State == UNQUOTED) {
       // Whitespace means the end of the token.
-      if (isWhitespace(Src[I])) {
+      if (isWhitespace(C)) {
         NewArgv.push_back(Saver.save(StringRef(Token)).data());
         Token.clear();
         State = INIT;
         // Mark the end of lines in response files
-        if (MarkEOLs && Src[I] == '\n')
+        if (MarkEOLs && C == '\n')
           NewArgv.push_back(nullptr);
         continue;
       }
-      if (Src[I] == '"') {
+      if (C == '"') {
         State = QUOTED;
         continue;
       }
-      if (Src[I] == '\\') {
+      if (C == '\\') {
         I = parseBackslash(Src, I, Token);
         continue;
       }
-      Token.push_back(Src[I]);
+      Token.push_back(C);
       continue;
     }
 
     // QUOTED state means that it's reading a token quoted by double quotes.
     if (State == QUOTED) {
-      if (Src[I] == '"') {
+      if (C == '"') {
         State = UNQUOTED;
         continue;
       }
-      if (Src[I] == '\\') {
+      if (C == '\\') {
         I = parseBackslash(Src, I, Token);
         continue;
       }
-      Token.push_back(Src[I]);
+      Token.push_back(C);
     }
   }
   // Append the last token after hitting EOF with no whitespace.