Implement C23 memalignment

Add the C23 memalignment function (query the alignment of a pointer)
to glibc.

Given how simple this operation is, it would make sense for compilers
to inline calls to this function, but I'm treating that as a compiler
matter (compilers should add it as a built-in function) rather than
adding an inline version to glibc headers (although such an inline
version would be reasonable as well).  I've filed
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122117 for this feature
in GCC.

Tested for x86_64 and x86.

5 files changed, 80 insertions, 0 deletions

diff --git a/stdlib/Makefile b/stdlib/Makefile
index d2c594885a..8d9a98602c 100644
--- a/stdlib/Makefile
+++ b/stdlib/Makefile
@@ -97,6 +97,7 @@ routines := \
   mblen \
   mbstowcs \
   mbtowc \
+  memalignment \
   mrand48 \
   mrand48_r \
   nrand48 \
@@ -299,6 +300,7 @@ tests := \
   tst-makecontext-align \
   tst-makecontext2 \
   tst-makecontext3 \
+  tst-memalignment \
   tst-on_exit \
   tst-qsort \
   tst-qsort2 \
@@ -441,6 +443,8 @@ CFLAGS-tst-stdc_bit_width.c += -fno-builtin
 CFLAGS-tst-stdc_bit_floor.c += -fno-builtin
 CFLAGS-tst-stdc_bit_ceil.c += -fno-builtin
 
+CFLAGS-tst-memalignment.c += -fno-builtin
+
 ifeq ($(have-cxx-thread_local),yes)
 CFLAGS-tst-quick_exit.o = -std=c++11
 LDLIBS-tst-quick_exit = -lstdc++
diff --git a/stdlib/Versions b/stdlib/Versions
index 6d024000f8..da7019a7ed 100644
--- a/stdlib/Versions
+++ b/stdlib/Versions
@@ -229,6 +229,9 @@ libc {
     ulabs;
     ullabs;
   }
+  GLIBC_2.43 {
+    memalignment;
+  }
   GLIBC_PRIVATE {
     # functions which have an additional interface since they are
     # are cancelable.
diff --git a/stdlib/memalignment.c b/stdlib/memalignment.c
new file mode 100644
index 0000000000..fb3eef96fe
--- /dev/null
+++ b/stdlib/memalignment.c
@@ -0,0 +1,27 @@
+/* Return the alignment of a pointer.
+   Copyright (C) 2025 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 <stdlib.h>
+
+/* Return the alignment of P.  */
+size_t
+memalignment (const void *p)
+{
+  size_t i = (size_t) p;
+  return i & -i;
+}
diff --git a/stdlib/stdlib.h b/stdlib/stdlib.h
index 1d6a83a22c..8d4d194a46 100644
--- a/stdlib/stdlib.h
+++ b/stdlib/stdlib.h
@@ -1164,6 +1164,9 @@ extern int ttyslot (void) __THROW;
 /* Call function __FUNC exactly once, even if invoked from several threads.
    All calls must be made with the same __FLAGS object.  */
 extern void call_once (once_flag *__flag, void (*__func)(void));
+
+/* Return the alignment of P.  */
+extern size_t memalignment (const void *__p);
 #endif
 
 #include <bits/stdlib-float.h>
diff --git a/stdlib/tst-memalignment.c b/stdlib/tst-memalignment.c
new file mode 100644
index 0000000000..518b83cc6b
--- /dev/null
+++ b/stdlib/tst-memalignment.c
@@ -0,0 +1,43 @@
+/* Test memalignment.
+   Copyright (C) 2025 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 <stdlib.h>
+#include <array_length.h>
+#include <support/check.h>
+
+static int
+do_test (void)
+{
+  void *null = NULL;
+  TEST_COMPARE (memalignment (NULL), 0);
+  TEST_COMPARE (memalignment (null), 0);
+  char ca[256];
+  array_foreach (p, ca)
+    TEST_VERIFY (memalignment (p) >= 1);
+  TEST_VERIFY (memalignment (&ca[0]) == 1 || memalignment (&ca[1]) == 1);
+  TEST_VERIFY (memalignment (&ca[0]) == 2 || memalignment (&ca[1]) == 2
+	       || memalignment (&ca[2]) == 2 || memalignment (&ca[3]) == 2);
+  long long int lla[256];
+  array_foreach (p, lla)
+    TEST_VERIFY (memalignment (p) >= _Alignof (long long int));
+  TEST_VERIFY (memalignment (&lla[0]) <= sizeof (long long int)
+	       || memalignment (&lla[1]) <= sizeof (long long int));
+  return 0;
+}
+
+#include <support/test-driver.c>