[ADT] Add a single-character version of the small vector split routine

on StringRef. Finding and splitting on a single character is
substantially faster than doing it on even a single character StringRef
-- we immediately get to a *very* tuned memchr call this way.

Even nicer, we get to this even in a debug build, shaving 18% off the
runtime of TripleTest.Normalization, helping PR23676 some more.

llvm-svn: 247244

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/unittests/ADT/StringRefTest.cpp b/llvm/unittests/ADT/StringRefTest.cpp
index d80179bd787..8af07da24ea 100644
--- a/llvm/unittests/ADT/StringRefTest.cpp
+++ b/llvm/unittests/ADT/StringRefTest.cpp
@@ -225,6 +225,11 @@ TEST(StringRefTest, Split2) {
   expected.push_back("a"); expected.push_back("b"); expected.push_back("c");
   StringRef("a,,b,c").split(parts, ",", 3, false);
   EXPECT_TRUE(parts == expected);
+
+  expected.clear(); parts.clear();
+  expected.push_back("a"); expected.push_back("b"); expected.push_back("c");
+  StringRef("a,,b,c").split(parts, ',', 3, false);
+  EXPECT_TRUE(parts == expected);
 }
 
 TEST(StringRefTest, Trim) {