Portable strncpy()/stpncpy().

Rather than just have the trivial byte-by-byte implementations, defer to
strnlen()/memcpy()/memset().

Use these for arm/arm64, and riscv64. x86-64 already has sse2 assembler
for these.

Change-Id: Ibc3193dd32d72df00cff953a5adeb8c8ec1c96dc
diff --git a/libc/Android.bp b/libc/Android.bp
index 4c35797..971c2b3 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -527,22 +527,17 @@
     arch: {
         arm: {
             srcs: [
-                "upstream-openbsd/lib/libc/string/stpncpy.c",
                 "upstream-openbsd/lib/libc/string/strncat.c",
                 "upstream-openbsd/lib/libc/string/strncmp.c",
-                "upstream-openbsd/lib/libc/string/strncpy.c",
             ],
         },
         arm64: {
             srcs: [
-                "upstream-openbsd/lib/libc/string/stpncpy.c",
                 "upstream-openbsd/lib/libc/string/strncat.c",
-                "upstream-openbsd/lib/libc/string/strncpy.c",
             ],
         },
         riscv64: {
             srcs: [
-                "upstream-openbsd/lib/libc/string/stpncpy.c",
             ],
         },
         x86: {
diff --git a/libc/bionic/string.cpp b/libc/bionic/string.cpp
index 1a05189..482bf3e 100644
--- a/libc/bionic/string.cpp
+++ b/libc/bionic/string.cpp
@@ -58,6 +58,24 @@
 }
 #endif
 
+#if defined(__arm__) || defined(__aarch64__) || defined(__riscv)
+__attribute__((__flatten__))
+char* stpncpy(char* dst, const char* src, size_t dst_n) {
+  size_t src_n = strnlen(src, dst_n);
+  memcpy(dst, src, src_n);
+  memset(dst + src_n, 0, dst_n - src_n);
+  return dst + src_n;
+}
+#endif
+
+#if defined(__arm__) || defined(__aarch64__)
+__attribute__((__flatten__))
+char* strncpy(char* dst, const char* src, size_t n) {
+  stpncpy(dst, src, n);
+  return dst;
+}
+#endif
+
 // If you have a fast memchr(), but not a strnlen().
 #if defined(__x86_64__)
 size_t strnlen(const char* s, size_t n) {
diff --git a/libc/upstream-openbsd/lib/libc/string/stpncpy.c b/libc/upstream-openbsd/lib/libc/string/stpncpy.c
deleted file mode 100644
index 6bdee5d..0000000
--- a/libc/upstream-openbsd/lib/libc/string/stpncpy.c
+++ /dev/null
@@ -1,57 +0,0 @@
-/*	$OpenBSD: stpncpy.c,v 1.3 2015/08/31 02:53:57 guenther Exp $	*/
-
-/*-
- * Copyright (c) 1990 The Regents of the University of California.
- * All rights reserved.
- *
- * This code is derived from software contributed to Berkeley by
- * 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 <string.h>
-
-char *
-stpncpy(char *dst, const char *src, size_t n)
-{
-	if (n != 0) {
-		char *d = dst;
-		const char *s = src;
-
-		dst = &dst[n];
-		do {
-			if ((*d++ = *s++) == 0) {
-				dst = d - 1;
-				/* NUL pad the remaining n-1 bytes */
-				while (--n != 0)
-					*d++ = 0;
-				break;
-			}
-		} while (--n != 0);
-	}
-	return (dst);
-}
-DEF_WEAK(stpncpy);
diff --git a/libc/upstream-openbsd/lib/libc/string/strncpy.c b/libc/upstream-openbsd/lib/libc/string/strncpy.c
deleted file mode 100644
index d6d8647..0000000
--- a/libc/upstream-openbsd/lib/libc/string/strncpy.c
+++ /dev/null
@@ -1,59 +0,0 @@
-/*	$OpenBSD: strncpy.c,v 1.8 2015/08/31 02:53:57 guenther Exp $	*/
-
-/*-
- * Copyright (c) 1990 The Regents of the University of California.
- * All rights reserved.
- *
- * This code is derived from software contributed to Berkeley by
- * 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 <string.h>
-
-/*
- * Copy src to dst, truncating or null-padding to always copy n bytes.
- * Return dst.
- */
-char *
-strncpy(char *dst, const char *src, size_t n)
-{
-	if (n != 0) {
-		char *d = dst;
-		const char *s = src;
-
-		do {
-			if ((*d++ = *s++) == 0) {
-				/* NUL pad the remaining n-1 bytes */
-				while (--n != 0)
-					*d++ = 0;
-				break;
-			}
-		} while (--n != 0);
-	}
-	return (dst);
-}
-DEF_STRONG(strncpy);