Bug 1376323 - Unroll the loop in nsTSubstring_CharT::StripTaggedASCII() in order to speed up StripCRLF(); r=froydnj

xpcom/string/nsTSubstring.cpp

@@ -951,19 +951,52 @@ nsTSubstring_CharT::StripTaggedASCII(const ASCIIMaskArray& aToStrip)
     AllocFailed(mLength);
   }
 
+  // What follows is an unrolled loop.  The unroll factor is small here (2),
+  // so we could structure the unrolling here in a more ad-hoc fashion, but
+  // keeping the unrolling more systematic seems to be a better choice both
+  // to keep the unrolled version a bit more readable, and also more easier
+  // to extend in the future if a change to the main body of the loop leads
+  // to an increase in the stall factor.
+  static const size_t UNROLL_FACTOR = 2;
+  size_t iterations = mLength / UNROLL_FACTOR;
+  const size_t left = mLength % UNROLL_FACTOR;
+
   char_type* to   = mData;
   char_type* from = mData;
-  char_type* end  = mData + mLength;
 
-  while (from < end) {
-    uint32_t theChar = (uint32_t)*from++;
+  // First, run the unrolled loop for the number of iterations that the main
+  // loop can be run without caring about the remainder runs.
+  while (iterations--) {
+    uint32_t firstChar = (uint32_t)*from++;
+    uint32_t secondChar = (uint32_t)*from++;
     // Replacing this with a call to ASCIIMask::IsMasked
     // regresses performance somewhat, so leaving it inlined.
-    if (!mozilla::ASCIIMask::IsMasked(aToStrip, theChar)) {
+    if (!mozilla::ASCIIMask::IsMasked(aToStrip, firstChar)) {
       // Not stripped, copy this char.
-      *to++ = (char_type)theChar;
+      *to++ = (char_type)firstChar;
+    }
+    if (!mozilla::ASCIIMask::IsMasked(aToStrip, secondChar)) {
+      *to++ = (char_type)secondChar;
     }
   }
+
+
+  // Now, deal with left-overs of what the main iterations didn't get to process.
+  switch (left) {
+  case 1:
+    {
+      uint32_t firstChar = (uint32_t)*from++;
+      if (!mozilla::ASCIIMask::IsMasked(aToStrip, firstChar)) {
+        *to++ = (char_type)firstChar;
+      }
+    }
+    MOZ_FALLTHROUGH;
+  case 0:
+  default:
+    // Nothing left!
+    break;
+  }
+
   *to = char_type(0); // add the null
   mLength = to - mData;
 }