Optimize the computation of 16-bit checksum

This commit was SVN r20965.

opal/util/crc.h

@@ -104,17 +104,26 @@ opal_csum(const void *  source, size_t csumlen)
     size_t lastPartialLength = 0;
     return opal_csum_partial(source, csumlen, &lastPartialLong, &lastPartialLength);
 }
-
+/*
+ * The buffer passed to this function is assumed to be 16-bit aligned
+ */
 static inline uint16_t
 opal_csum16 (const void *  source, size_t csumlen)
 {
-    unsigned char *src = (unsigned char *) source;
-    uint16_t csum = 0;
-    size_t i;
+    uint16_t *src = (uint16_t *) source;
+    register uint32_t csum = 0;
 
-	for(i = 0; i < csumlen; i++) {
+    while (csumlen > 1) {
 	    csum += *src++;
-	}
+        csumlen -= 2;
+    }
+    /* Add leftover byte, if any */ 
+    if(csumlen > 0)
+        csum += *((unsigned char*)src);
+    /* Fold 32-bit checksum to 16 bits */
+    while(csum >> 16) {
+        csum = (csum & 0xFFFF) + (csum >> 16);    
+    }        
     return csum;
 }