Index: clang/lib/Basic/SourceManager.cpp
===================================================================
--- clang/lib/Basic/SourceManager.cpp
+++ clang/lib/Basic/SourceManager.cpp
@@ -1256,6 +1256,11 @@
 #include <emmintrin.h>
 #endif
 
+static constexpr inline bool hasless(unsigned long x, unsigned long n) {
+  // See http://graphics.stanford.edu/~seander/bithacks.html#HasLessInWord
+  return (((x)-~0UL/255*(n))&~(x)&~0UL/255*128);
+}
+
 LineOffsetMapping LineOffsetMapping::get(llvm::MemoryBufferRef Buffer,
                                          llvm::BumpPtrAllocator &Alloc) {
   // Find the file offsets of all of the *physical* source lines.  This does
@@ -1269,9 +1274,44 @@
   const unsigned char *End = (const unsigned char *)Buffer.getBufferEnd();
   const std::size_t BufLen = End - Buf;
   unsigned I = 0;
+  unsigned long Word;
+
+  constexpr char NewLineBound = std::max('\r', '\n');
+
+  // scan sizeof(Word) bytes at a time for new lines.
+  // This is much faster than scanning each byte independently.
+  if(BufLen > sizeof(Word)) {
+    while (I < BufLen - sizeof(Word)) {
+      memcpy(&Word, Buf + I, sizeof(Word));
+      // no new line => jump over sizeof(Word) bytes.
+      if(!hasless(Word, 1 + NewLineBound)) {
+        I += sizeof(Word);
+        continue;
+      }
+
+      // Otherwise scan for the next newline - and we know there's at least one.
+      do {
+        if (Buf[I] == '\n') {
+          LineOffsets.push_back(I + 1);
+	  ++I;
+	  break;
+        } else if (Buf[I] == '\r') {
+          // If this is \r\n, skip both characters.
+          if (I + 1 < BufLen && Buf[I + 1] == '\n')
+            ++I;
+          LineOffsets.push_back(I + 1);
+	  ++I;
+	  break;
+        }
+        ++I;
+      } while(1);
+    }
+  }
+
+  // Handle tail using a regular check.
   while (I < BufLen) {
     // Use a fast check to catch both newlines
-    if (LLVM_UNLIKELY(Buf[I] <= std::max('\n', '\r'))) {
+    if (LLVM_UNLIKELY(Buf[I] <= NewLineBound)) {
       if (Buf[I] == '\n') {
         LineOffsets.push_back(I + 1);
       } else if (Buf[I] == '\r') {