From 2f032c84a2cbed37308da8b29e84efb6eb2fe9e5 Mon Sep 17 00:00:00 2001
From: Michael Brown <mcb30@ipxe.org>
Date: Tue, 7 Jul 2020 13:13:28 +0100
Subject: [PATCH] [libc] Provide an unoptimised generic_memcpy_reverse()

Signed-off-by: Michael Brown <mcb30@ipxe.org>
---
 src/core/string.c    | 29 ++++++++++++++++++++++-------
 src/include/string.h |  2 ++
 2 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/src/core/string.c b/src/core/string.c
index 5bd9dae8b..c35015b59 100644
--- a/src/core/string.c
+++ b/src/core/string.c
@@ -52,7 +52,7 @@ void * generic_memset ( void *dest, int character, size_t len ) {
 }
 
 /**
- * Copy memory region
+ * Copy memory region (forwards)
  *
  * @v dest		Destination region
  * @v src		Source region
@@ -68,6 +68,23 @@ void * generic_memcpy ( void *dest, const void *src, size_t len ) {
 	return dest;
 }
 
+/**
+ * Copy memory region (backwards)
+ *
+ * @v dest		Destination region
+ * @v src		Source region
+ * @v len		Length
+ * @ret dest		Destination region
+ */
+void * generic_memcpy_reverse ( void *dest, const void *src, size_t len ) {
+	const uint8_t *src_bytes = ( src + len );
+	uint8_t *dest_bytes = ( dest + len );
+
+	while ( len-- )
+		*(--dest_bytes) = *(--src_bytes);
+	return dest;
+}
+
 /**
  * Copy (possibly overlapping) memory region
  *
@@ -77,14 +94,12 @@ void * generic_memcpy ( void *dest, const void *src, size_t len ) {
  * @ret dest		Destination region
  */
 void * generic_memmove ( void *dest, const void *src, size_t len ) {
-	const uint8_t *src_bytes = ( src + len );
-	uint8_t *dest_bytes = ( dest + len );
 
-	if ( dest < src )
+	if ( dest < src ) {
 		return generic_memcpy ( dest, src, len );
-	while ( len-- )
-		*(--dest_bytes) = *(--src_bytes);
-	return dest;
+	} else {
+		return generic_memcpy_reverse ( dest, src, len );
+	}
 }
 
 /**
diff --git a/src/include/string.h b/src/include/string.h
index 0f4182001..5f5aecb92 100644
--- a/src/include/string.h
+++ b/src/include/string.h
@@ -15,6 +15,8 @@ extern void * generic_memset ( void *dest, int character,
 			       size_t len ) __nonnull;
 extern void * generic_memcpy ( void *dest, const void *src,
 			       size_t len ) __nonnull;
+extern void * generic_memcpy_reverse ( void *dest, const void *src,
+				       size_t len ) __nonnull;
 extern void * generic_memmove ( void *dest, const void *src,
 				size_t len ) __nonnull;