Reduce the overhead of the memcpy check

This is based on patch from John Reiser <jreiser@bitwagon.com>. Rather
than use _buildin_abs (which also has issues with 64-bit values) do
the test as two conditional tests. Also add the unlikely() notation to give
the compiler some hint on how to organize the code to make common
case straight-line code.

1 files changed, 5 insertions, 2 deletions

diff --git a/memstomp.c b/memstomp.c
index 69faeaa..fdf8853 100644
--- a/memstomp.c
+++ b/memstomp.c
@@ -44,6 +44,9 @@
 #include <signal.h>
 #define ABRT_TRAP raise(SIGSEGV)
 
+#define likely(x)	__builtin_expect(!!(x), 1)
+#define unlikely(x)	__builtin_expect(!!(x), 0)
+
 static bool abrt_trap = false;
 
 #ifndef SCHED_RESET_ON_FORK
@@ -294,10 +297,10 @@ static void warn_memcpy(void * dest, const void * src, size_t count)
 
 void * memcpy(void * dest, const void * src, size_t count)
 {
-	size_t distance = __builtin_abs(((char *)dest - (char *)src));
+	size_t d = (char *)dest - (char *)src;
 	
 	/* Check for overlap. */
-	if (distance < count) {
+	if (unlikely(d < count || -d < count)) {
 		if (abrt_trap) ABRT_TRAP;
 		/* report the overlap */
 		warn_memcpy(dest, src, count);