4 files changed, 58 insertions, 14 deletions
diff --git a/absl/strings/BUILD.bazel b/absl/strings/BUILD.bazel
index 13badb7b9c25..1e52312b47c6 100644
--- a/absl/strings/BUILD.bazel
+++ b/absl/strings/BUILD.bazel
@@ -84,6 +84,7 @@ cc_library(
         "internal/utf8.cc",
     ],
     hdrs = [
+        "internal/bits.h",
         "internal/char_map.h",
         "internal/ostringstream.h",
         "internal/resize_uninitialized.h",
diff --git a/absl/strings/CMakeLists.txt b/absl/strings/CMakeLists.txt
index 83cb934dba9d..9dc47328c731 100644
--- a/absl/strings/CMakeLists.txt
+++ b/absl/strings/CMakeLists.txt
@@ -31,6 +31,7 @@ list(APPEND STRINGS_PUBLIC_HEADERS
 
 
 list(APPEND STRINGS_INTERNAL_HEADERS
+  "internal/bits.h"
   "internal/char_map.h"
   "internal/memutil.h"
   "internal/ostringstream.h"
diff --git a/absl/strings/internal/bits.h b/absl/strings/internal/bits.h
new file mode 100644
index 000000000000..901082ccddfc
--- /dev/null
+++ b/absl/strings/internal/bits.h
@@ -0,0 +1,53 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_BITS_H_
+#define ABSL_STRINGS_INTERNAL_BITS_H_
+
+#include <cstdint>
+
+#if defined(_MSC_VER) && defined(_M_X64)
+#include <intrin.h>
+#pragma intrinsic(_BitScanReverse64)
+#endif
+
+namespace absl {
+namespace strings_internal {
+
+// Returns the number of leading 0 bits in a 64-bit value.
+inline int CountLeadingZeros64(uint64_t n) {
+#if defined(__GNUC__)
+  static_assert(sizeof(unsigned long long) == sizeof(n),  // NOLINT(runtime/int)
+                "__builtin_clzll does not take 64bit arg");
+  return n == 0 ? 64 : __builtin_clzll(n);
+#elif defined(_MSC_VER) && defined(_M_X64)
+  unsigned long result;  // NOLINT(runtime/int)
+  if (_BitScanReverse64(&result, n)) {
+    return 63 - result;
+  }
+  return 64;
+#else
+  int zeroes = 60;
+  if (n >> 32) zeroes -= 32, n >>= 32;
+  if (n >> 16) zeroes -= 16, n >>= 16;
+  if (n >> 8) zeroes -= 8, n >>= 8;
+  if (n >> 4) zeroes -= 4, n >>= 4;
+  return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0\0"[n] + zeroes;
+#endif
+}
+
+}  // namespace strings_internal
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_BITS_H_
diff --git a/absl/strings/numbers.cc b/absl/strings/numbers.cc
index b4140b3605bc..68ef7999a981 100644
--- a/absl/strings/numbers.cc
+++ b/absl/strings/numbers.cc
@@ -32,6 +32,7 @@
 
 #include "absl/base/internal/raw_logging.h"
 #include "absl/strings/ascii.h"
+#include "absl/strings/internal/bits.h"
 #include "absl/strings/internal/memutil.h"
 #include "absl/strings/str_cat.h"
 
@@ -302,18 +303,6 @@ char* numbers_internal::FastIntToBuffer(int64_t i, char* buffer) {
   return numbers_internal::FastIntToBuffer(u, buffer);
 }
 
-// Returns the number of leading 0 bits in a 64-bit value.
-// TODO(jorg): Replace with builtin_clzll if available.
-// Are we shipping util/bits in absl?
-static inline int CountLeadingZeros64(uint64_t n) {
-  int zeroes = 60;
-  if (n >> 32) zeroes -= 32, n >>= 32;
-  if (n >> 16) zeroes -= 16, n >>= 16;
-  if (n >> 8) zeroes -= 8, n >>= 8;
-  if (n >> 4) zeroes -= 4, n >>= 4;
-  return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0\0"[n] + zeroes;
-}
-
 // Given a 128-bit number expressed as a pair of uint64_t, high half first,
 // return that number multiplied by the given 32-bit value.  If the result is
 // too large to fit in a 128-bit number, divide it by 2 until it fits.
@@ -351,7 +340,7 @@ static std::pair<uint64_t, uint64_t> Mul32(std::pair<uint64_t, uint64_t> num,
   uint64_t bits128_up = (bits96_127 >> 32) + (bits64_127 < bits64_95);
   if (bits128_up == 0) return {bits64_127, bits0_63};
 
-  int shift = 64 - CountLeadingZeros64(bits128_up);
+  int shift = 64 - strings_internal::CountLeadingZeros64(bits128_up);
   uint64_t lo = (bits0_63 >> shift) + (bits64_127 << (64 - shift));
   uint64_t hi = (bits64_127 >> shift) + (bits128_up << (64 - shift));
   return {hi, lo};
@@ -382,7 +371,7 @@ static std::pair<uint64_t, uint64_t> PowFive(uint64_t num, int expfive) {
       5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5,
       5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5};
   result = Mul32(result, powers_of_five[expfive & 15]);
-  int shift = CountLeadingZeros64(result.first);
+  int shift = strings_internal::CountLeadingZeros64(result.first);
   if (shift != 0) {
     result.first = (result.first << shift) + (result.second >> (64 - shift));
     result.second = (result.second << shift);