[PATCH] D133660: [Support] Add fast path for StringRef::find with needle of length 2.

[Tatsuyuki Ishi via Phabricator via llvm-commits]

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InclusionRewriter on Windows (CRLF line endings) will exercise this in a
hot path. Calling memcmp repeatedly would be highly suboptimal for that
use case, so give it a specialized path.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D133660

Files:
  llvm/lib/Support/StringRef.cpp


Index: llvm/lib/Support/StringRef.cpp
===================================================================
--- llvm/lib/Support/StringRef.cpp
+++ llvm/lib/Support/StringRef.cpp
@@ -148,6 +148,23 @@
 
   const char *Stop = Start + (Size - N + 1);
 
+  if (N == 2) {
+    // Provide a fast path for newline finding (CRLF case) in InclusionRewriter.
+    // This is basically a naive search with a little bit twiddling.
+
+    // In theory we could use uint16_t, but some architectures don't support
+    // 16-bit arithmetic and will require a (& 65535) after the bit operations.
+    // Using 32-bit arithmetic with a shift of 16 would be more efficient for
+    // those cases.
+    uint32_t NeedleWord =
+        (uint32_t)(uint8_t)Needle[0] << 16 | (uint8_t)Needle[1];
+    uint32_t HaystackWord =
+        (uint32_t)(uint8_t)Start[0] << 16 | (uint8_t)Start[1];
+    for (Start++; Start < Stop && NeedleWord != HaystackWord;)
+      HaystackWord = HaystackWord << 16 | (uint8_t) * ++Start;
+    return NeedleWord == HaystackWord ? Start - Data - 1 : npos;
+  }
+
   // For short haystacks or unsupported needles fall back to the naive algorithm
   if (Size < 16 || N > 255) {
     do {


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