[libc][NFC] remove dependency on non standard ssize_t

`ssize_t` is from POSIX and is not standard unfortunately.
Rewritting the code so it doesn't depend on it.

Differential Revision: https://reviews.llvm.org/D94760

Author: gchatelet

libc/src/string/memmove.cpp

@@ -11,21 +11,25 @@
 #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)
+static inline void move_byte_forward(char *dest_m, const char *src_m,
+                                     size_t count) {
+  for (size_t offset = 0; count; --count, ++offset)
+    dest_m[offset] = src_m[offset];
+}
+
+static inline void move_byte_backward(char *dest_m, const char *src_m,
+                                      size_t count) {
+  for (size_t offset = count - 1; count; --count, --offset)
     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);
+  char *dest_c = reinterpret_cast<char *>(dest);
+  const char *src_c = reinterpret_cast<const 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.
@@ -50,11 +54,11 @@ LLVM_LIBC_FUNCTION(void *, memmove,
   // src_c : [___abcde_]  [_abcde___]
   // dest_c: [_abc--___]  [___--cde_]
 
-  // TODO: Optimize `move_byte(...)` function.
+  // TODO: Optimize `move_byte_xxx(...)` functions.
   if (dest_c < src_c)
-    move_byte(dest_c, src_c, count, /*pointer add*/ 1);
+    move_byte_forward(dest_c, src_c, count);
   if (dest_c > src_c)
-    move_byte(dest_c + count - 1, src_c + count - 1, count, /*pointer add*/ -1);
+    move_byte_backward(dest_c, src_c, count);
   return dest;
 }