string: Add fallback implementation for ctz/clz

The libgcc implementations of __builtin_clzl/__builtin_ctzl may require
access to additional data that is not marked as hidden, which could
introduce additional GOT indirection and necessitate RELATIVE relocs.
And the RELATIVE reloc is an issue if the code is used during static-pie
startup before self-relocation (for instance, during an assert).

For this case, the ABI can add a string-bitops.h header that defines
HAVE_BITOPTS_WORKING to 0. A configure check for this issue is tricky
because it requires linking against the standard libraries, which
create many RELATIVE relocations and complicate filtering those that
might be created by the builtins.

The fallback is disabled by default, so no target is affected.

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>

diff --git a/sysdeps/generic/string-bitops.h b/sysdeps/generic/string-bitops.h
new file mode 100644 (file)
index 0000000..c4c03fb
--- /dev/null
+++ b/sysdeps/generic/string-bitops.h
@@ -0,0 +1,26 @@
+/* Zero byte detection, define whether to use stdbit.h
+   Copyright (C) 2026 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
+   <http://www.gnu.org/licenses/>.  */
+
+/* These macros define whether index_first/index_last macro can use the
+   stdbit.h routines, otherwise fallback that do not call libgcc is used
+   instead.
+
+   An architecture can override it is it uses a generic string routines
+   on process startup and it can not use the libgcc routines for find
+   trailing/leading zeros.  */
+#define HAVE_BITOPTS_WORKING 1

diff --git a/sysdeps/generic/string-fzi.h b/sysdeps/generic/string-fzi.h
index 9f74e42dbf64786899bc7be0721a79694c6be0d5..10be1e2e34d798e0d8e472f987b999f725a3db4c 100644 (file)
--- a/sysdeps/generic/string-fzi.h
+++ b/sysdeps/generic/string-fzi.h
@@ -22,23 +22,39 @@
 #include <limits.h>
 #include <endian.h>
 #include <string-fza.h>
+#include <string-bitops.h>
+#include <stdbit.h>
 
-static __always_inline int
-clz (find_t c)
+static __always_inline unsigned int
+ctzb (find_t c)
 {
-  if (sizeof (find_t) == sizeof (unsigned long))
-    return __builtin_clzl (c);
+#if HAVE_BITOPTS_WORKING
+  return stdc_trailing_zeros (c) / CHAR_BIT;
+#else
+  if (sizeof (find_t) <= 4)
+    return (((c & -c) >> 7) * 0x00010203) >> 24;
   else
-    return __builtin_clzll (c);
+    return (((c & -c) >> 7) * 0x0001020304050607UL) >> 56;
+#endif
 }
 
-static __always_inline int
-ctz (find_t c)
+static __always_inline unsigned int
+clzb (find_t c)
 {
-  if (sizeof (find_t) == sizeof (unsigned long))
-    return __builtin_ctzl (c);
-  else
-    return __builtin_ctzll (c);
+#if HAVE_BITOPTS_WORKING
+  return stdc_leading_zeros (c) / CHAR_BIT;
+#else
+#  if ULONG_MAX == 0xFFFFFFFFUL
+  c |= c >> 8;
+  c |= c >> 16;
+  return ((c >> 7) * 0x30f0f0f0) >> 28;
+#  else
+  c |= c >> 8;
+  c |= c >> 16;
+  c |= c >> 32;
+  return ((c >> 7) * 0x70f0f0f0f0f0f0f0UL) >> 60;
+#  endif
+#endif
 }
 
 /* A subroutine for the index_zero functions.  Given a test word C, return
@@ -47,12 +63,12 @@ ctz (find_t c)
 static __always_inline unsigned int
 index_first (find_t c)
 {
-  int r;
+  unsigned int r;
   if (__BYTE_ORDER == __LITTLE_ENDIAN)
-    r = ctz (c);
+    r = ctzb (c);
   else
-    r = clz (c);
-  return r / CHAR_BIT;
+    r = clzb (c);
+  return r;
 }
 
 /* Similarly, but return the (memory order) index of the last byte that is
@@ -60,12 +76,12 @@ index_first (find_t c)
 static __always_inline unsigned int
 index_last (find_t c)
 {
-  int r;
+  unsigned int r;
   if (__BYTE_ORDER == __LITTLE_ENDIAN)
-    r = clz (c);
+    r = clzb (c);
   else
-    r = ctz (c);
-  return sizeof (find_t) - 1 - (r / CHAR_BIT);
+    r = ctzb (c);
+  return sizeof (find_t) - 1 - r;
 }
 
 #endif /* STRING_FZI_H */