diff --git a/libc/arch-riscv64/string/memset.c b/libc/arch-riscv64/string/memset.c
new file mode 100644
index 0000000..d51cbf9
--- /dev/null
+++ b/libc/arch-riscv64/string/memset.c
@@ -0,0 +1,108 @@
+/*-
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright (c) 1990, 1993
+ *	The Regents of the University of California.  All rights reserved.
+ *
+ * This code is derived from software contributed to Berkeley by
+ * Mike Hibler and Chris Torek.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the University nor the names of its contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/types.h>
+
+#include <limits.h>
+
+#define	wsize	sizeof(u_long)
+#define	wmask	(wsize - 1)
+
+#include <string.h>
+
+#define	RETURN	return (dst0)
+#define	VAL	c0
+#define	WIDEVAL	c
+
+void *
+memset_gc(void *dst0, int c0, size_t length)
+{
+	size_t t;
+	u_long c;
+	u_char *dst;
+
+	dst = dst0;
+	/*
+	 * If not enough words, just fill bytes.  A length >= 2 words
+	 * guarantees that at least one of them is `complete' after
+	 * any necessary alignment.  For instance:
+	 *
+	 *	|-----------|-----------|-----------|
+	 *	|00|01|02|03|04|05|06|07|08|09|0A|00|
+	 *	          ^---------------------^
+	 *		 dst		 dst+length-1
+	 *
+	 * but we use a minimum of 3 here since the overhead of the code
+	 * to do word writes is substantial.
+	 *
+	 * TODO: This threshold might not be sensible for 64-bit u_long.
+	 * We should benchmark and revisit this decision.
+	 */
+	if (length < 3 * wsize) {
+		while (length != 0) {
+			*dst++ = VAL;
+			--length;
+		}
+		RETURN;
+	}
+
+	if ((c = (u_char)c0) != 0) {	/* Fill the word. */
+		c = (c << 8) | c;	/* u_long is 16 bits. */
+		c = (c << 16) | c;	/* u_long is 32 bits. */
+		c = (c << 32) | c;	/* u_long is 64 bits. */
+	}
+	/* Align destination by filling in bytes. */
+	if ((t = (long)dst & wmask) != 0) {
+		t = wsize - t;
+		length -= t;
+		do {
+			*dst++ = VAL;
+		} while (--t != 0);
+	}
+
+	/* Fill words.  Length was >= 2*words so we know t >= 1 here. */
+	t = length / wsize;
+	do {
+		*(u_long *)(void *)dst = WIDEVAL;
+		dst += wsize;
+	} while (--t != 0);
+
+	/* Mop up trailing bytes, if any. */
+	t = length & wmask;
+	if (t != 0)
+		do {
+			*dst++ = VAL;
+		} while (--t != 0);
+	RETURN;
+}