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-rw-r--r--bits/byteswap.h25
1 files changed, 20 insertions, 5 deletions
diff --git a/bits/byteswap.h b/bits/byteswap.h
index b193ea19ec..ec2051ace2 100644
--- a/bits/byteswap.h
+++ b/bits/byteswap.h
@@ -53,12 +53,27 @@ __bswap_32 (unsigned int __bsx)
 
 #if defined __GNUC__ && __GNUC__ >= 2
 /* Swap bytes in 64 bit value.  */
+#define __bswap_constant_64(x) \
+     ((((x) & 0xff00000000000000ull) >> 56)				      \
+      | (((x) & 0x00ff000000000000ull) >> 40)				      \
+      | (((x) & 0x0000ff0000000000ull) >> 24)				      \
+      | (((x) & 0x000000ff00000000ull) >> 8)				      \
+      | (((x) & 0x00000000ff000000ull) << 8)				      \
+      | (((x) & 0x0000000000ff0000ull) << 24)				      \
+      | (((x) & 0x000000000000ff00ull) << 40)				      \
+      | (((x) & 0x00000000000000ffull) << 56))
+
 # define __bswap_64(x) \
      (__extension__							      \
-      ({ union { unsigned long long int __ll;				      \
-		 unsigned int __l[2]; } __v, __r;			      \
-	 __v.__ll = (x);						      \
-	 __r.__l[0] = __bswap_32 (__v.__l[1]);				      \
-	 __r.__l[1] = __bswap_32 (__v.__l[0]);				      \
+      ({ union { __extension__ unsigned long long int __ll;		      \
+		 unsigned long int __l[2]; } __w, __r;			      \
+         if (__builtin_constant_p (x))					      \
+	   __r.__ll = __bswap_constant_64 (x);				      \
+	 else								      \
+	   {								      \
+	     __w.__ll = (x);						      \
+	     __r.__l[0] = __bswap_32 (__w.__l[1]);			      \
+	     __r.__l[1] = __bswap_32 (__w.__l[0]);			      \
+	   }								      \
 	 __r.__ll; }))
 #endif