libc: implement C23 <stdbit.h> functions

This new header complies with ISO/IEC 9899:2024 (C23).

Contrary to glibc, we do not provide inline definitions in
<stdbit.h> as we expect our system compiler to soon recognise
these as builtins anyway.

Relnotes:	yes
MFC after:	1 month
Reviewed by:	adrian
Approved by:	markj (mentor)
Differential Revision:	https://reviews.freebsd.org/D53657

(cherry picked from commit 6296500a85c8474e3ff3fe2f8e4a9d56dd0acd64)

1 files changed, 57 insertions, 0 deletions

diff --git a/lib/libc/stdbit/stdc_first_leading_zero.c b/lib/libc/stdbit/stdc_first_leading_zero.c
new file mode 100644
index 000000000000..e6609b51b648
--- /dev/null
+++ b/lib/libc/stdbit/stdc_first_leading_zero.c
@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <limits.h>
+#include <stdbit.h>
+
+unsigned int
+stdc_first_leading_zero_uc(unsigned char x)
+{
+	const int offset = UINT_WIDTH - UCHAR_WIDTH;
+
+	if (x == UCHAR_MAX)
+		return (0);
+
+	return (__builtin_clz(~(unsigned int)x << offset) + 1);
+}
+
+unsigned int
+stdc_first_leading_zero_us(unsigned short x)
+{
+	const int offset = UINT_WIDTH - USHRT_WIDTH;
+
+	if (x == USHRT_MAX)
+		return (0);
+
+	return (__builtin_clz(~(unsigned int)x << offset) + 1);
+}
+
+unsigned int
+stdc_first_leading_zero_ui(unsigned int x)
+{
+	if (x == ~0U)
+		return (0);
+
+	return (__builtin_clz(~x) + 1);
+}
+
+unsigned int
+stdc_first_leading_zero_ul(unsigned long x)
+{
+	if (x == ~0UL)
+		return (0);
+
+	return (__builtin_clzl(~x) + 1);
+}
+
+unsigned int
+stdc_first_leading_zero_ull(unsigned long long x)
+{
+	if (x == ~0ULL)
+		return (0);
+
+	return (__builtin_clzll(~x) + 1);
+}