1 files changed, 53 insertions, 0 deletions
diff --git a/absl/strings/internal/bits.h b/absl/strings/internal/bits.h
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+++ 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_