[libc] Add memcmp implementation.

Reviewed By: gchatelet

Differential Revision: https://reviews.llvm.org/D93009

libc/config/linux/aarch64/entrypoints.txt

@@ -26,6 +26,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     # string.h entrypoints
     libc.src.string.bzero
     libc.src.string.memchr
+    libc.src.string.memcmp
     libc.src.string.memcpy
     libc.src.string.memset
     libc.src.string.memrchr

libc/config/linux/x86_64/entrypoints.txt

@@ -48,6 +48,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     # string.h entrypoints
     libc.src.string.bzero
     libc.src.string.memchr
+    libc.src.string.memcmp
     libc.src.string.memcpy
     libc.src.string.memrchr
     libc.src.string.memset

libc/src/string/CMakeLists.txt

@@ -58,6 +58,14 @@ add_entrypoint_object(
     .string_utils
 )
 
+add_entrypoint_object(
+  memcmp
+  SRCS
+    memcmp.cpp
+  HDRS
+    memcmp.h
+)
+
 add_entrypoint_object(
   strchr
   SRCS

libc/src/string/memcmp.cpp

@@ -0,0 +1,27 @@
+//===-- 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 <stddef.h> // size_t
+
+namespace __llvm_libc {
+
+// TODO: It is a simple implementation, an optimized version is preparing.
+int LLVM_LIBC_ENTRYPOINT(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;
+}
+
+} // namespace __llvm_libc

libc/src/string/memcmp.h

@@ -0,0 +1,20 @@
+//===-- Implementation header for memcmp ------------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STRING_MEMCMP_H
+#define LLVM_LIBC_SRC_STRING_MEMCMP_H
+
+#include <stddef.h> // size_t
+
+namespace __llvm_libc {
+
+int memcmp(const void *lhs, const void *rhs, size_t count);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STRING_MEMCMP_H

libc/test/src/string/CMakeLists.txt

@@ -52,6 +52,16 @@ add_libc_unittest(
     libc.src.string.memchr
 )
 
+add_libc_unittest(
+  memcmp_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    memcmp_test.cpp
+  DEPENDS
+    libc.src.string.memcmp
+)
+
 add_libc_unittest(
   strchr_test
   SUITE

libc/test/src/string/memcmp_test.cpp

@@ -0,0 +1,34 @@
+//===-- Unittests for 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 "utils/UnitTest/Test.h"
+
+TEST(MemcmpTest, CmpZeroByte) {
+  const char *lhs = "ab";
+  const char *rhs = "bc";
+  EXPECT_EQ(__llvm_libc::memcmp(lhs, rhs, 0), 0);
+}
+
+TEST(MemcmpTest, LhsRhsAreTheSame) {
+  const char *lhs = "ab";
+  const char *rhs = "ab";
+  EXPECT_EQ(__llvm_libc::memcmp(lhs, rhs, 2), 0);
+}
+
+TEST(MemcmpTest, LhsBeforeRhsLexically) {
+  const char *lhs = "ab";
+  const char *rhs = "ac";
+  EXPECT_EQ(__llvm_libc::memcmp(lhs, rhs, 2), -1);
+}
+
+TEST(MemcmpTest, LhsAfterRhsLexically) {
+  const char *lhs = "ac";
+  const char *rhs = "ab";
+  EXPECT_EQ(__llvm_libc::memcmp(lhs, rhs, 2), 1);
+}