Re: [PATCH v2] xdiff: load full words in the inner loop of xdl_hash_record

Thomas Rast [off-list ref] writes:

+unsigned long xdl_hash_record(char const **data, char const *top, long flags) {
+	unsigned long hash = 5381;
+	unsigned long a = 0, mask = 0;
+	char const *ptr = *data;
+	char const *end = top-sizeof(unsigned long)+1;
+
+	if (flags & XDF_WHITESPACE_FLAGS)
+		return xdl_hash_record_with_whitespace(data, top, flags);
+
+	ptr -= sizeof(unsigned long);
+	do {
+		hash += hash << 5;
+		hash ^= a;
+		ptr += sizeof(unsigned long);
+		if (ptr >= end)
+			break;
+		a = *(unsigned long *)ptr;
+		/* Do we have any '\n' bytes in this word? */
+		mask = has_zero(a ^ NEWLINEBYTES);
+	} while (!mask);
+
+	if (ptr >= end) {
+		/*
+		 * There is only a partial word left at the end of the
+		 * buffer. Because we may work with a memory mapping,
+		 * we have to grab the rest byte by byte instead of
+		 * blindly reading it.

Ok, because end is sufficiently below "top" and because you know "top-1"
should be a mapped byte, the "ptr >= end" check in the above loop ensures
that reading ul at ptr will stay within the range of mapped pages.

Cute but ugly ;-)