1 files changed, 60 insertions, 0 deletions
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));
+}