string: Add internal memswap implementation

The prototype is:

  void __memswap (void *restrict p1, void *restrict p2, size_t n)

The function swaps the content of two memory blocks P1 and P2 of
len N.  Memory overlap is NOT handled.

It will be used on qsort optimization.

Checked on x86_64-linux-gnu and aarch64-linux-gnu.
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>

1 files changed, 41 insertions, 0 deletions

diff --git a/sysdeps/generic/memswap.h b/sysdeps/generic/memswap.h
new file mode 100644
index 0000000..f09dae1
--- /dev/null
+++ b/sysdeps/generic/memswap.h
@@ -0,0 +1,41 @@
+/* Swap the content of two memory blocks, overlap is NOT handled.
+   Copyright (C) 2023 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <string.h>
+
+static inline void
+__memswap (void *__restrict p1, void *__restrict p2, size_t n)
+{
+  /* Use multiple small memcpys with constant size to enable inlining on most
+     targets.  */
+  enum { SWAP_GENERIC_SIZE = 32 };
+  unsigned char tmp[SWAP_GENERIC_SIZE];
+  while (n > SWAP_GENERIC_SIZE)
+    {
+      memcpy (tmp, p1, SWAP_GENERIC_SIZE);
+      p1 = __mempcpy (p1, p2, SWAP_GENERIC_SIZE);
+      p2 = __mempcpy (p2, tmp, SWAP_GENERIC_SIZE);
+      n -= SWAP_GENERIC_SIZE;
+    }
+  while (n > 0)
+    {
+      unsigned char t = ((unsigned char *)p1)[--n];
+      ((unsigned char *)p1)[n] = ((unsigned char *)p2)[n];
+      ((unsigned char *)p2)[n] = t;
+    }
+}