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)

17 files changed, 847 insertions, 0 deletions

diff --git a/lib/libc/Makefile b/lib/libc/Makefile
index 8705568f6d34..34474cfa9fe4 100644
--- a/lib/libc/Makefile
+++ b/lib/libc/Makefile
@@ -118,6 +118,7 @@ NOASM=
 .endif
 .include "${LIBC_SRCTOP}/regex/Makefile.inc"
 .include "${LIBC_SRCTOP}/resolv/Makefile.inc"
+.include "${LIBC_SRCTOP}/stdbit/Makefile.inc"
 .include "${LIBC_SRCTOP}/stdio/Makefile.inc"
 .include "${LIBC_SRCTOP}/stdlib/Makefile.inc"
 .include "${LIBC_SRCTOP}/stdtime/Makefile.inc"
diff --git a/lib/libc/stdbit/Makefile.inc b/lib/libc/stdbit/Makefile.inc
new file mode 100644
index 000000000000..2ea96d2b6dfe
--- /dev/null
+++ b/lib/libc/stdbit/Makefile.inc
@@ -0,0 +1,19 @@
+# sources for <stdbit.h>
+.PATH: ${LIBC_SRCTOP}/stdbit
+
+SRCS+=	stdc_bit_ceil.c \
+	stdc_bit_floor.c \
+	stdc_bit_width.c \
+	stdc_count_ones.c \
+	stdc_count_zeros.c \
+	stdc_first_leading_one.c \
+	stdc_first_leading_zero.c \
+	stdc_first_trailing_one.c \
+	stdc_first_trailing_zero.c \
+	stdc_has_single_bit.c \
+	stdc_leading_ones.c \
+	stdc_leading_zeros.c \
+	stdc_trailing_ones.c \
+	stdc_trailing_zeros.c
+
+SYM_MAPS+=${LIBC_SRCTOP}/stdbit/Symbol.map
diff --git a/lib/libc/stdbit/Symbol.map b/lib/libc/stdbit/Symbol.map
new file mode 100644
index 000000000000..7014d6984c07
--- /dev/null
+++ b/lib/libc/stdbit/Symbol.map
@@ -0,0 +1,85 @@
+FBSD_1.9 {
+	stdc_leading_zeros_uc;
+	stdc_leading_zeros_us;
+	stdc_leading_zeros_ui;
+	stdc_leading_zeros_ul;
+	stdc_leading_zeros_ull;
+
+	stdc_leading_ones_uc;
+	stdc_leading_ones_us;
+	stdc_leading_ones_ui;
+	stdc_leading_ones_ul;
+	stdc_leading_ones_ull;
+
+	stdc_trailing_zeros_uc;
+	stdc_trailing_zeros_us;
+	stdc_trailing_zeros_ui;
+	stdc_trailing_zeros_ul;
+	stdc_trailing_zeros_ull;
+
+	stdc_trailing_ones_uc;
+	stdc_trailing_ones_us;
+	stdc_trailing_ones_ui;
+	stdc_trailing_ones_ul;
+	stdc_trailing_ones_ull;
+
+	stdc_first_leading_zero_uc;
+	stdc_first_leading_zero_us;
+	stdc_first_leading_zero_ui;
+	stdc_first_leading_zero_ul;
+	stdc_first_leading_zero_ull;
+
+	stdc_first_leading_one_uc;
+	stdc_first_leading_one_us;
+	stdc_first_leading_one_ui;
+	stdc_first_leading_one_ul;
+	stdc_first_leading_one_ull;
+
+	stdc_first_trailing_zero_uc;
+	stdc_first_trailing_zero_us;
+	stdc_first_trailing_zero_ui;
+	stdc_first_trailing_zero_ul;
+	stdc_first_trailing_zero_ull;
+
+	stdc_first_trailing_one_uc;
+	stdc_first_trailing_one_us;
+	stdc_first_trailing_one_ui;
+	stdc_first_trailing_one_ul;
+	stdc_first_trailing_one_ull;
+
+	stdc_count_zeros_uc;
+	stdc_count_zeros_us;
+	stdc_count_zeros_ui;
+	stdc_count_zeros_ul;
+	stdc_count_zeros_ull;
+
+	stdc_count_ones_uc;
+	stdc_count_ones_us;
+	stdc_count_ones_ui;
+	stdc_count_ones_ul;
+	stdc_count_ones_ull;
+
+	stdc_has_single_bit_uc;
+	stdc_has_single_bit_us;
+	stdc_has_single_bit_ui;
+	stdc_has_single_bit_ul;
+	stdc_has_single_bit_ull;
+
+	stdc_bit_width_uc;
+	stdc_bit_width_us;
+	stdc_bit_width_ui;
+	stdc_bit_width_ul;
+	stdc_bit_width_ull;
+
+	stdc_bit_floor_uc;
+	stdc_bit_floor_us;
+	stdc_bit_floor_ui;
+	stdc_bit_floor_ul;
+	stdc_bit_floor_ull;
+
+	stdc_bit_ceil_uc;
+	stdc_bit_ceil_us;
+	stdc_bit_ceil_ui;
+	stdc_bit_ceil_ul;
+	stdc_bit_ceil_ull;
+};
diff --git a/lib/libc/stdbit/stdc_bit_ceil.c b/lib/libc/stdbit/stdc_bit_ceil.c
new file mode 100644
index 000000000000..2dfd7bbcbea2
--- /dev/null
+++ b/lib/libc/stdbit/stdc_bit_ceil.c
@@ -0,0 +1,71 @@
+/*
+ * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <assert.h>
+#include <limits.h>
+#include <stdbit.h>
+
+/* Ensure we don't shift 1U out of range. */
+static_assert(UCHAR_WIDTH < UINT_WIDTH,
+    "stdc_bit_ceil_uc needs UCHAR_WIDTH < UINT_WIDTH");
+
+unsigned char
+stdc_bit_ceil_uc(unsigned char x)
+{
+	if (x <= 1)
+		return (1);
+
+	return (1U << (UINT_WIDTH - __builtin_clz(x - 1)));
+}
+
+/* Ensure we don't shift 1U out of range. */
+static_assert(USHRT_WIDTH < UINT_WIDTH,
+    "stdc_bit_ceil_us needs USHRT_WIDTH < UINT_WIDTH");
+
+unsigned short
+stdc_bit_ceil_us(unsigned short x)
+{
+	if (x <= 1)
+		return (1);
+
+	return (1U << (UINT_WIDTH - __builtin_clz(x - 1)));
+}
+
+unsigned int
+stdc_bit_ceil_ui(unsigned int x)
+{
+	if (x <= 1)
+		return (1);
+
+	if (x > UINT_MAX/2 + 1)
+		return (0);
+
+	return (1U << (UINT_WIDTH - __builtin_clz(x - 1)));
+}
+
+unsigned long
+stdc_bit_ceil_ul(unsigned long x)
+{
+	if (x <= 1)
+		return (1);
+
+	if (x > ULONG_MAX/2 + 1)
+		return (0);
+
+	return (1UL << (ULONG_WIDTH - __builtin_clzl(x - 1)));
+}
+
+unsigned long long
+stdc_bit_ceil_ull(unsigned long long x)
+{
+	if (x <= 1)
+		return (1);
+
+	if (x > ULLONG_MAX/2 + 1)
+		return (0);
+
+	return (1ULL << (ULLONG_WIDTH - __builtin_clzll(x - 1)));
+}
diff --git a/lib/libc/stdbit/stdc_bit_floor.c b/lib/libc/stdbit/stdc_bit_floor.c
new file mode 100644
index 000000000000..0a491d6ec7b3
--- /dev/null
+++ b/lib/libc/stdbit/stdc_bit_floor.c
@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <limits.h>
+#include <stdbit.h>
+
+unsigned char
+stdc_bit_floor_uc(unsigned char x)
+{
+	if (x == 0)
+		return (0);
+
+	return (1U << (UINT_WIDTH - __builtin_clz(x) - 1));
+}
+
+unsigned short
+stdc_bit_floor_us(unsigned short x)
+{
+	if (x == 0)
+		return (0);
+
+	return (1U << (UINT_WIDTH - __builtin_clz(x) - 1));
+}
+
+unsigned int
+stdc_bit_floor_ui(unsigned int x)
+{
+	if (x == 0)
+		return (0);
+
+	return (1U << (UINT_WIDTH - __builtin_clz(x) - 1));
+}
+
+unsigned long
+stdc_bit_floor_ul(unsigned long x)
+{
+	if (x == 0)
+		return (0);
+
+	return (1UL << (ULONG_WIDTH - __builtin_clzl(x) - 1));
+}
+
+unsigned long long
+stdc_bit_floor_ull(unsigned long long x)
+{
+	if (x == 0)
+		return (0);
+
+	return (1ULL << (ULLONG_WIDTH - __builtin_clzll(x) - 1));
+}
diff --git a/lib/libc/stdbit/stdc_bit_width.c b/lib/libc/stdbit/stdc_bit_width.c
new file mode 100644
index 000000000000..e248fae48e9f
--- /dev/null
+++ b/lib/libc/stdbit/stdc_bit_width.c
@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <limits.h>
+#include <stdbit.h>
+
+unsigned int
+stdc_bit_width_uc(unsigned char x)
+{
+	if (x == 0)
+		return (0);
+
+	return (UINT_WIDTH - __builtin_clz(x));
+}
+
+unsigned int
+stdc_bit_width_us(unsigned short x)
+{
+	if (x == 0)
+		return (0);
+
+	return (UINT_WIDTH - __builtin_clz(x));
+}
+
+unsigned int
+stdc_bit_width_ui(unsigned int x)
+{
+	if (x == 0)
+		return (0);
+
+	return (UINT_WIDTH - __builtin_clz(x));
+}
+
+unsigned int
+stdc_bit_width_ul(unsigned long x)
+{
+	if (x == 0)
+		return (0);
+
+	return (ULONG_WIDTH - __builtin_clzl(x));
+}
+
+unsigned int
+stdc_bit_width_ull(unsigned long long x)
+{
+	if (x == 0)
+		return (0);
+
+	return (ULLONG_WIDTH - __builtin_clzll(x));
+}
diff --git a/lib/libc/stdbit/stdc_count_ones.c b/lib/libc/stdbit/stdc_count_ones.c
new file mode 100644
index 000000000000..c5198dca84df
--- /dev/null
+++ b/lib/libc/stdbit/stdc_count_ones.c
@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <limits.h>
+#include <stdbit.h>
+
+unsigned int
+stdc_count_ones_uc(unsigned char x)
+{
+	return (__builtin_popcount(x));
+}
+
+unsigned int
+stdc_count_ones_us(unsigned short x)
+{
+	return (__builtin_popcount(x));
+}
+
+unsigned int
+stdc_count_ones_ui(unsigned int x)
+{
+	return (__builtin_popcount(x));
+}
+
+unsigned int
+stdc_count_ones_ul(unsigned long x)
+{
+	return (__builtin_popcountl(x));
+}
+
+unsigned int
+stdc_count_ones_ull(unsigned long long x)
+{
+	return (__builtin_popcountll(x));
+}
diff --git a/lib/libc/stdbit/stdc_count_zeros.c b/lib/libc/stdbit/stdc_count_zeros.c
new file mode 100644
index 000000000000..35db965afcb5
--- /dev/null
+++ b/lib/libc/stdbit/stdc_count_zeros.c
@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <limits.h>
+#include <stdbit.h>
+
+unsigned int
+stdc_count_zeros_uc(unsigned char x)
+{
+	return (__builtin_popcount(x ^ UCHAR_MAX));
+}
+
+unsigned int
+stdc_count_zeros_us(unsigned short x)
+{
+	return (__builtin_popcount(x ^ USHRT_MAX));
+}
+
+unsigned int
+stdc_count_zeros_ui(unsigned int x)
+{
+	return (__builtin_popcount(~x));
+}
+
+unsigned int
+stdc_count_zeros_ul(unsigned long x)
+{
+	return (__builtin_popcountl(~x));
+}
+
+unsigned int
+stdc_count_zeros_ull(unsigned long long x)
+{
+	return (__builtin_popcountll(~x));
+}
diff --git a/lib/libc/stdbit/stdc_first_leading_one.c b/lib/libc/stdbit/stdc_first_leading_one.c
new file mode 100644
index 000000000000..de436856569f
--- /dev/null
+++ b/lib/libc/stdbit/stdc_first_leading_one.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_one_uc(unsigned char x)
+{
+	const int offset = UINT_WIDTH - UCHAR_WIDTH;
+
+	if (x == 0)
+		return (0);
+
+	return (__builtin_clz(x << offset) + 1);
+}
+
+unsigned int
+stdc_first_leading_one_us(unsigned short x)
+{
+	const int offset = UINT_WIDTH - USHRT_WIDTH;
+
+	if (x == 0)
+		return (0);
+
+	return (__builtin_clz(x << offset) + 1);
+}
+
+unsigned int
+stdc_first_leading_one_ui(unsigned int x)
+{
+	if (x == 0)
+		return (0);
+
+	return (__builtin_clz(x) + 1);
+}
+
+unsigned int
+stdc_first_leading_one_ul(unsigned long x)
+{
+	if (x == 0)
+		return (0);
+
+	return (__builtin_clzl(x) + 1);
+}
+
+unsigned int
+stdc_first_leading_one_ull(unsigned long long x)
+{
+	if (x == 0)
+		return (0);
+
+	return (__builtin_clzll(x) + 1);
+}
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);
+}
diff --git a/lib/libc/stdbit/stdc_first_trailing_one.c b/lib/libc/stdbit/stdc_first_trailing_one.c
new file mode 100644
index 000000000000..22972758d25d
--- /dev/null
+++ b/lib/libc/stdbit/stdc_first_trailing_one.c
@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <stdbit.h>
+
+unsigned int
+stdc_first_trailing_one_uc(unsigned char x)
+{
+	if (x == 0)
+		return (0);
+
+	return (__builtin_ctz(x) + 1);
+}
+
+unsigned int
+stdc_first_trailing_one_us(unsigned short x)
+{
+	if (x == 0)
+		return (0);
+
+	return (__builtin_ctz(x) + 1);
+}
+
+unsigned int
+stdc_first_trailing_one_ui(unsigned int x)
+{
+	if (x == 0)
+		return (0);
+
+	return (__builtin_ctz(x) + 1);
+}
+
+unsigned int
+stdc_first_trailing_one_ul(unsigned long x)
+{
+	if (x == 0)
+		return (0);
+
+	return (__builtin_ctzl(x) + 1);
+}
+
+unsigned int
+stdc_first_trailing_one_ull(unsigned long long x)
+{
+	if (x == 0)
+		return (0);
+
+	return (__builtin_ctzll(x) + 1);
+}
diff --git a/lib/libc/stdbit/stdc_first_trailing_zero.c b/lib/libc/stdbit/stdc_first_trailing_zero.c
new file mode 100644
index 000000000000..27395c6977c3
--- /dev/null
+++ b/lib/libc/stdbit/stdc_first_trailing_zero.c
@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <limits.h>
+#include <stdbit.h>
+
+unsigned int
+stdc_first_trailing_zero_uc(unsigned char x)
+{
+	if (x == UCHAR_MAX)
+		return (0);
+
+	return (__builtin_ctz(~x) + 1);
+}
+
+unsigned int
+stdc_first_trailing_zero_us(unsigned short x)
+{
+	if (x == USHRT_MAX)
+		return (0);
+
+	return (__builtin_ctz(~x) + 1);
+}
+
+unsigned int
+stdc_first_trailing_zero_ui(unsigned int x)
+{
+	if (x == ~0U)
+		return (0);
+
+	return (__builtin_ctz(~x) + 1);
+}
+
+unsigned int
+stdc_first_trailing_zero_ul(unsigned long x)
+{
+	if (x == ~0UL)
+		return (0);
+
+	return (__builtin_ctzl(~x) + 1);
+}
+
+unsigned int
+stdc_first_trailing_zero_ull(unsigned long long x)
+{
+	if (x == ~0ULL)
+		return (0);
+
+	return (__builtin_ctzll(~x) + 1);
+}
diff --git a/lib/libc/stdbit/stdc_has_single_bit.c b/lib/libc/stdbit/stdc_has_single_bit.c
new file mode 100644
index 000000000000..e5f676265551
--- /dev/null
+++ b/lib/libc/stdbit/stdc_has_single_bit.c
@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <stdbit.h>
+#include <stdbool.h>
+
+bool
+stdc_has_single_bit_uc(unsigned char x)
+{
+	return (x != 0 && (x & x - 1) == 0);
+}
+
+bool
+stdc_has_single_bit_us(unsigned short x)
+{
+	return (x != 0 && (x & x - 1) == 0);
+}
+
+bool
+stdc_has_single_bit_ui(unsigned int x)
+{
+	return (x != 0 && (x & x - 1) == 0);
+}
+
+bool
+stdc_has_single_bit_ul(unsigned long x)
+{
+	return (x != 0 && (x & x - 1) == 0);
+}
+
+bool
+stdc_has_single_bit_ull(unsigned long long x)
+{
+	return (x != 0 && (x & x - 1) == 0);
+}
diff --git a/lib/libc/stdbit/stdc_leading_ones.c b/lib/libc/stdbit/stdc_leading_ones.c
new file mode 100644
index 000000000000..79d3ec7f2065
--- /dev/null
+++ b/lib/libc/stdbit/stdc_leading_ones.c
@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <assert.h>
+#include <limits.h>
+#include <stdbit.h>
+
+/* Avoid triggering undefined behavior if x == 0. */
+static_assert(UCHAR_WIDTH < UINT_WIDTH,
+    "stdc_leading_ones_uc needs UCHAR_WIDTH < UINT_WIDTH");
+
+unsigned int
+stdc_leading_ones_uc(unsigned char x)
+{
+	const int offset = UINT_WIDTH - UCHAR_WIDTH;
+
+	return (__builtin_clz(~(x << offset)));
+}
+
+/* Avoid triggering undefined behavior if x == 0. */
+static_assert(USHRT_WIDTH < UINT_WIDTH,
+    "stdc_leading_ones_us needs USHRT_WIDTH < UINT_WIDTH");
+
+unsigned int
+stdc_leading_ones_us(unsigned short x)
+{
+	const int offset = UINT_WIDTH - USHRT_WIDTH;
+
+	return (__builtin_clz(~(x << offset)));
+}
+
+unsigned int
+stdc_leading_ones_ui(unsigned int x)
+{
+	if (x == ~0U)
+		return (UINT_WIDTH);
+
+	return (__builtin_clz(~x));
+}
+
+unsigned int
+stdc_leading_ones_ul(unsigned long x)
+{
+	if (x == ~0UL)
+		return (ULONG_WIDTH);
+
+	return (__builtin_clzl(~x));
+}
+
+unsigned int
+stdc_leading_ones_ull(unsigned long long x)
+{
+	if (x == ~0ULL)
+		return (ULLONG_WIDTH);
+
+	return (__builtin_clzll(~x));
+}
diff --git a/lib/libc/stdbit/stdc_leading_zeros.c b/lib/libc/stdbit/stdc_leading_zeros.c
new file mode 100644
index 000000000000..2fdf64ec93d4
--- /dev/null
+++ b/lib/libc/stdbit/stdc_leading_zeros.c
@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <assert.h>
+#include <limits.h>
+#include <stdbit.h>
+
+/* Offset must be greater than zero. */
+static_assert(UCHAR_WIDTH < UINT_WIDTH,
+    "stdc_leading_zeros_uc needs UCHAR_WIDTH < UINT_WIDTH");
+
+unsigned int
+stdc_leading_zeros_uc(unsigned char x)
+{
+	const int offset = UINT_WIDTH - UCHAR_WIDTH;
+
+	return (__builtin_clz((x << offset) + (1U << (offset - 1))));
+}
+
+/* Offset must be greater than zero. */
+static_assert(USHRT_WIDTH < UINT_WIDTH,
+    "stdc_leading_zeros_us needs USHRT_WIDTH < UINT_WIDTH");
+
+unsigned int
+stdc_leading_zeros_us(unsigned short x)
+{
+	const int offset = UINT_WIDTH - USHRT_WIDTH;
+
+	return (__builtin_clz((x << offset) + (1U << (offset - 1))));
+}
+
+unsigned int
+stdc_leading_zeros_ui(unsigned int x)
+{
+	if (x == 0)
+		return (UINT_WIDTH);
+
+	return (__builtin_clz(x));
+}
+
+unsigned int
+stdc_leading_zeros_ul(unsigned long x)
+{
+	if (x == 0)
+		return (ULONG_WIDTH);
+
+	return (__builtin_clzl(x));
+}
+
+unsigned int
+stdc_leading_zeros_ull(unsigned long long x)
+{
+	if (x == 0)
+		return (ULLONG_WIDTH);
+
+	return (__builtin_clzll(x));
+}
diff --git a/lib/libc/stdbit/stdc_trailing_ones.c b/lib/libc/stdbit/stdc_trailing_ones.c
new file mode 100644
index 000000000000..c1a0e6b991da
--- /dev/null
+++ b/lib/libc/stdbit/stdc_trailing_ones.c
@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <assert.h>
+#include <limits.h>
+#include <stdbit.h>
+
+/* Avoid triggering undefined behavior if x == ~0. */
+static_assert(UCHAR_WIDTH < UINT_WIDTH,
+    "stdc_trailing_ones_uc needs UCHAR_WIDTH < UINT_WIDTH");
+
+unsigned int
+stdc_trailing_ones_uc(unsigned char x)
+{
+	return (__builtin_ctz(~x));
+}
+
+/* Avoid triggering undefined behavior if x == ~0. */
+static_assert(USHRT_WIDTH < UINT_WIDTH,
+    "stdc_trailing_ones_uc needs USHRT_WIDTH < UINT_WIDTH");
+
+unsigned int
+stdc_trailing_ones_us(unsigned short x)
+{
+	return (__builtin_ctz(~x));
+}
+
+unsigned int
+stdc_trailing_ones_ui(unsigned int x)
+{
+	if (x == ~0U)
+		return (UINT_WIDTH);
+
+	return (__builtin_ctz(~x));
+}
+
+unsigned int
+stdc_trailing_ones_ul(unsigned long x)
+{
+	if (x == ~0UL)
+		return (ULONG_WIDTH);
+
+	return (__builtin_ctzl(~x));
+}
+
+unsigned int
+stdc_trailing_ones_ull(unsigned long long x)
+{
+	if (x == ~0ULL)
+		return (ULLONG_WIDTH);
+
+	return (__builtin_ctzll(~x));
+}
diff --git a/lib/libc/stdbit/stdc_trailing_zeros.c b/lib/libc/stdbit/stdc_trailing_zeros.c
new file mode 100644
index 000000000000..6213f62cffdb
--- /dev/null
+++ b/lib/libc/stdbit/stdc_trailing_zeros.c
@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <assert.h>
+#include <limits.h>
+#include <stdbit.h>
+
+/* Ensure we do not shift 1U out of range. */
+static_assert(UCHAR_WIDTH < UINT_WIDTH,
+    "stdc_trailing_zeros_uc needs UCHAR_WIDTH < UINT_WIDTH");
+
+unsigned int
+stdc_trailing_zeros_uc(unsigned char x)
+{
+	return (__builtin_ctz(x | 1U << UCHAR_WIDTH));
+}
+
+/* Ensure we do not shift 1U out of range. */
+static_assert(USHRT_WIDTH < UINT_WIDTH,
+    "stdc_trailing_zeros_uc needs USHRT_WIDTH < UINT_WIDTH");
+
+unsigned int
+stdc_trailing_zeros_us(unsigned short x)
+{
+	return (__builtin_ctz(x | 1U << USHRT_WIDTH));
+}
+
+unsigned int
+stdc_trailing_zeros_ui(unsigned int x)
+{
+	if (x == 0U)
+		return (UINT_WIDTH);
+
+	return (__builtin_ctz(x));
+}
+
+unsigned int
+stdc_trailing_zeros_ul(unsigned long x)
+{
+	if (x == 0UL)
+		return (ULONG_WIDTH);
+
+	return (__builtin_ctzl(x));
+}
+
+unsigned int
+stdc_trailing_zeros_ull(unsigned long long x)
+{
+	if (x == 0ULL)
+		return (ULLONG_WIDTH);
+
+	return (__builtin_ctzll(x));
+}