[libc] Add memcmp implementation.

[cheng.w]

libc/src/string/memcmp.cpp

15

There is a TODO in strcmp.cpp (reviews.llvm.org/D82134) about speeding up. I did not find out the answer. Should I leave a TODO here too?

[sivachandra]

This simple implementation is probably OK to start with. But, I will add @gchatelet as he might also be having plans to add optimized versions of memcmp.

[gchatelet]

libc/src/string/memcmp.cpp

15

Yes please, let's add the TODO in the meantime (I have an optimized version to contribute).

22

[nit] No need for the dot / Capitalization.
count is 0 or _lhs and _rhs are the same.

[cheng.w]

Thanks Siva, have a nice holiday!

[cheng.w]

Thanks Chatelet, I thought you started holiday.
Add TODO, fix nit.

libc/config/linux/aarch64/entrypoints.txt

set(TARGET_LIBC_ENTRYPOINTS
     libc.src.ctype.toupper
     
     # errno.h entrypoints
     libc.src.errno.__errno_location
 
     # 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.src.string.strcat
     libc.src.string.strchr
     libc.src.string.strcpy
     libc.src.string.strcspn
     libc.src.string.strlen

libc/config/linux/x86_64/entrypoints.txt

set(TARGET_LIBC_ENTRYPOINTS
 
     # stdlib.h entrypoints
     libc.src.stdlib._Exit
     libc.src.stdlib.abort
 
     # 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.strcat
     libc.src.string.strchr
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn

libc/src/string/CMakeLists.txt

SRCS
     memchr.cpp
   HDRS
     memchr.h
   DEPENDS
     .string_utils
 )
 
 add_entrypoint_object(
+  memcmp
+  SRCS
+    memcmp.cpp
+  HDRS
+    memcmp.h
+)
+
+add_entrypoint_object(
   strchr
   SRCS
     strchr.cpp
   HDRS
     strchr.h
 )
 
 add_entrypoint_object(

libc/src/string/memcmp.h

+//===-- 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/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 <stddef.h> // size_t
+
+namespace __llvm_libc {
+
+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/test/src/string/CMakeLists.txt

SUITE
     libc_string_unittests
   SRCS
     memchr_test.cpp
   DEPENDS
     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_string_unittests
   SRCS
     strchr_test.cpp
   DEPENDS
     libc.src.string.strchr
 )

libc/test/src/string/memcmp_test.cpp

+//===-- 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);
+}