[libc] Add memmove implementation.

Use `memcpy` rather than copying bytes one by one, for there might be large
size structs to move.

Reviewed By: gchatelet, sivachandra

Differential Revision: https://reviews.llvm.org/D93195

Author: ChennngWang

libc/config/linux/aarch64/entrypoints.txt

@@ -30,6 +30,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.memchr
     libc.src.string.memcmp
     libc.src.string.memcpy
+    libc.src.string.memmove
     libc.src.string.memset
     libc.src.string.memrchr
     libc.src.string.strcat

libc/config/linux/x86_64/entrypoints.txt

@@ -52,6 +52,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.memchr
     libc.src.string.memcmp
     libc.src.string.memcpy
+    libc.src.string.memmove
     libc.src.string.memrchr
     libc.src.string.memset
     libc.src.string.strcat

libc/src/string/CMakeLists.txt

@@ -66,6 +66,18 @@ add_entrypoint_object(
     memcmp.h
 )
 
+add_entrypoint_object(
+  memmove
+  SRCS
+    memmove.cpp
+  HDRS
+    memmove.h
+  DEPENDS
+    libc.include.unistd
+    libc.src.stdlib.abs_utils
+    libc.src.string.memcpy
+)
+
 add_entrypoint_object(
   strchr
   SRCS

libc/src/string/memmove.cpp

@@ -0,0 +1,61 @@
+//===-- Implementation of memmove -----------------------------------------===//
+//
+// 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/memmove.h"
+#include "src/__support/common.h"
+#include "src/stdlib/abs_utils.h"
+#include "src/string/memcpy.h"
+#include <stddef.h> // size_t, ptrdiff_t
+#include <unistd.h> // ssize_t
+
+namespace __llvm_libc {
+
+// src_m and dest_m might be the beginning or end.
+static inline void move_byte(unsigned char *dest_m, const unsigned char *src_m,
+                             size_t count, ssize_t direction) {
+  for (ssize_t offset = 0; count; --count, offset += direction)
+    dest_m[offset] = src_m[offset];
+}
+
+LLVM_LIBC_FUNCTION(void *, memmove,
+                   (void *dest, const void *src, size_t count)) {
+  unsigned char *dest_c = reinterpret_cast<unsigned char *>(dest);
+  const unsigned char *src_c = reinterpret_cast<const unsigned char *>(src);
+
+  // If the distance between src_c and dest_c is equal to or greater
+  // than count (integer_abs(src_c - dest_c) >= count), they would not overlap.
+  // e.g.   greater     equal       overlapping
+  //        [12345678]  [12345678]  [12345678]
+  // src_c: [_ab_____]  [_ab_____]  [_ab_____]
+  // dest_c:[_____yz_]  [___yz___]  [__yz____]
+
+  // Use memcpy if src_c and dest_c do not overlap.
+  if (__llvm_libc::integer_abs(src_c - dest_c) >= static_cast<ptrdiff_t>(count))
+    return __llvm_libc::memcpy(dest_c, src_c, count);
+
+  // Overlap cases.
+  // If dest_c starts before src_c (dest_c < src_c), copy forward(pointer add 1)
+  // from beginning to end.
+  // If dest_c starts after src_c (dest_c > src_c), copy backward(pointer add
+  // -1) from end to beginning.
+  // If dest_c and src_c start at the same address (dest_c == src_c),
+  // just return dest.
+  // e.g.    forward      backward
+  //             *-->        <--*
+  // src_c : [___abcde_]  [_abcde___]
+  // dest_c: [_abc--___]  [___--cde_]
+
+  // TODO: Optimize `move_byte(...)` function.
+  if (dest_c < src_c)
+    move_byte(dest_c, src_c, count, /*pointer add*/ 1);
+  if (dest_c > src_c)
+    move_byte(dest_c + count - 1, src_c + count - 1, count, /*pointer add*/ -1);
+  return dest;
+}
+
+} // namespace __llvm_libc

libc/src/string/memmove.h

@@ -0,0 +1,20 @@
+//===-- Implementation header for memmove -----------------------*- 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_MEMMOVE_H
+#define LLVM_LIBC_SRC_STRING_MEMMOVE_H
+
+#include <stddef.h> // size_t
+
+namespace __llvm_libc {
+
+void *memmove(void *dest, const void *src, size_t count);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STRING_MEMMOVE_H

libc/test/src/string/CMakeLists.txt

@@ -62,6 +62,17 @@ add_libc_unittest(
     libc.src.string.memcmp
 )
 
+add_libc_unittest(
+  memmove_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    memmove_test.cpp
+  DEPENDS
+    libc.src.string.memcmp
+    libc.src.string.memmove
+)
+
 add_libc_unittest(
   strchr_test
   SUITE

libc/test/src/string/memmove_test.cpp

@@ -0,0 +1,70 @@
+//===-- Unittests for memmove ---------------------------------------------===//
+//
+// 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/string/memmove.h"
+#include "utils/CPP/ArrayRef.h"
+#include "utils/UnitTest/Test.h"
+
+class MemmoveTest : public __llvm_libc::testing::Test {
+public:
+  void check_memmove(void *dest, const void *src, size_t count, const void *str,
+                     const __llvm_libc::cpp::ArrayRef<unsigned char> expected) {
+    void *result = __llvm_libc::memmove(dest, src, count);
+    // Making sure the pointer returned is same with dest.
+    EXPECT_EQ(result, dest);
+    // expected is designed according to str.
+    // dest and src might be part of str.
+    // Making sure the str is same with expected.
+    EXPECT_EQ(__llvm_libc::memcmp(str, expected.data(), expected.size()), 0);
+  }
+};
+
+TEST_F(MemmoveTest, MoveZeroByte) {
+  unsigned char dest[] = {'a', 'b'};
+  const unsigned char src[] = {'y', 'z'};
+  const unsigned char expected[] = {'a', 'b'};
+  check_memmove(dest, src, 0, dest, expected);
+}
+
+TEST_F(MemmoveTest, OverlapThatDestAndSrcPointToSameAddress) {
+  unsigned char str[] = {'a', 'b'};
+  const unsigned char expected[] = {'a', 'b'};
+  check_memmove(str, str, 1, str, expected);
+}
+
+TEST_F(MemmoveTest, OverlapThatDestStartsBeforeSrc) {
+  // Set boundary at beginning and end for not overstepping when
+  // copy forward or backward.
+  unsigned char str[] = {'z', 'a', 'b', 'c', 'z'};
+  const unsigned char expected[] = {'z', 'b', 'c', 'c', 'z'};
+  // dest is &str[1].
+  check_memmove(&str[1], &str[2], 2, str, expected);
+}
+
+TEST_F(MemmoveTest, OverlapThatDestStartsAfterSrc) {
+  unsigned char str[] = {'z', 'a', 'b', 'c', 'z'};
+  const unsigned char expected[] = {'z', 'a', 'a', 'b', 'z'};
+  check_memmove(&str[2], &str[1], 2, str, expected);
+}
+
+// e.g. dest follow src.
+// str: [abcdefghij]
+//      [__src_____]
+//      [_____dest_]
+TEST_F(MemmoveTest, SrcFollowDest) {
+  unsigned char str[] = {'z', 'a', 'b', 'z'};
+  const unsigned char expected[] = {'z', 'b', 'b', 'z'};
+  check_memmove(&str[1], &str[2], 1, str, expected);
+}
+
+TEST_F(MemmoveTest, DestFollowSrc) {
+  unsigned char str[] = {'z', 'a', 'b', 'z'};
+  const unsigned char expected[] = {'z', 'a', 'a', 'z'};
+  check_memmove(&str[2], &str[1], 1, str, expected);
+}