[libc] Add an optimized version for memcmp

[gchatelet]

• Add missing newline at EOF
• Remove unaligned loop for size<384

libc/src/string/memcmp.cpp

 //===-- Implementation of memcmp ------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "src/string/memcmp.h"
 #include "src/__support/common.h"
+#include "src/string/memory_utils/elements.h"
+
 #include <stddef.h> // size_t
 
 namespace __llvm_libc {
 
-// TODO: It is a simple implementation, an optimized version is preparing.
+static int memcmp_impl(const char *lhs, const char *rhs, size_t count) {
+#if defined(__i386__) || defined(__x86_64__)
+  using namespace ::__llvm_libc::x86;
+#else
+  using namespace ::__llvm_libc::scalar;
+#endif
+
+  if (count == 0)
+    return 0;
+  if (count == 1)
+    return ThreeWayCompare<_1>(lhs, rhs);
+  if (count == 2)
+    return ThreeWayCompare<_2>(lhs, rhs);
+  if (count == 3)
+    return ThreeWayCompare<_3>(lhs, rhs);
+  if (count <= 8)
+    return ThreeWayCompare<HeadTail<_4>>(lhs, rhs, count);
+  if (count <= 16)
+    return ThreeWayCompare<HeadTail<_8>>(lhs, rhs, count);
+  if (count <= 32)
+    return ThreeWayCompare<HeadTail<_16>>(lhs, rhs, count);
+  if (count <= 64)
+    return ThreeWayCompare<HeadTail<_32>>(lhs, rhs, count);
+  if (count <= 128)
+    return ThreeWayCompare<HeadTail<_64>>(lhs, rhs, count);
+  return ThreeWayCompare<Align<_16>::Then<Loop<_32>>>(lhs, rhs, count);
+}
+
 LLVM_LIBC_FUNCTION(int, memcmp,
                    (const void *lhs, const void *rhs, size_t count)) {
-  const unsigned char *_lhs = reinterpret_cast<const unsigned char *>(lhs);
-  const unsigned char *_rhs = reinterpret_cast<const unsigned char *>(rhs);
-  for (size_t i = 0; i < count; ++i)
-    if (_lhs[i] != _rhs[i])
-      return _lhs[i] - _rhs[i];
-  // count is 0 or _lhs and _rhs are the same.
-  return 0;
+  return memcmp_impl(static_cast<const char *>(lhs),
+                     static_cast<const char *>(rhs), count);
 }
 
 } // namespace __llvm_libc