From: Jeff King <peff@peff.net>
Date: Wed, 04 Nov 2020 13:36:53 GMT
Subject: Re: [PATCH] pack-write: use hashwrite_be32() instead of double-buffering array
Message-ID: <20201104133653.GC3030480@coredump.intra.peff.net>
In-Reply-To: <aec69531-d621-ab26-efd4-96e1ae0ed3a4@web.de>

On Sun, Nov 01, 2020 at 09:52:12AM +0100, René Scharfe wrote:

> hashwrite() already buffers writes, so pass the fanout table entries
> individually via hashwrite_be32(), which also does the endianess
> conversion for us.  This avoids a memory copy, shortens the code and
> reduces the number of magic numbers.

Yep, this seems trivially correct. The key observation is that we are
filling the array in order:

> @@ -101,20 +100,19 @@ const char *write_idx_file(const char *index_name, struct pack_idx_entry **objec
>  	for (i = 0; i < 256; i++) {
>  		struct pack_idx_entry **next = list;
>  		while (next < last) {
>  			struct pack_idx_entry *obj = *next;
>  			if (obj->oid.hash[0] != i)
>  				break;
>  			next++;
>  		}
> -		array[i] = htonl(next - sorted_by_sha);
> +		hashwrite_be32(f, next - sorted_by_sha);
>  		list = next;
>  	}
> -	hashwrite(f, array, 256 * 4);

Perhaps obvious, but I got bit trying to do another similar conversion
recently that was filling in the array out-of-order (not on the list,
but in some improvements in the bitmap code that haven't been sent in
yet).

-Peff