Re: [PATCH v1 2/5] Teach git to optionally utilize a file system monitor to speed up detecting new or changed files.

From: Jeff King <peff@peff.net>
Date: May 16, 2017, 00:34 UTC
Message-ID: <20170516003414.yliltu5fsaudfhyu@sigill.intra.peff.net>
In-Reply-To: <20170516002214.tlqkk4zrwdzcdjha@genre.crustytoothpaste.net>

On Tue, May 16, 2017 at 12:22:14AM +0000, brian m. carlson wrote:

Show 17 quoted lines

> On Mon, May 15, 2017 at 03:13:44PM -0400, Ben Peart wrote:
> > +	istate->last_update = (time_t)ntohll(*(uint64_t *)index);
> > +	index += sizeof(uint64_t);
> > +
> > +	ewah_size = ntohl(*(uint32_t *)index);
> > +	index += sizeof(uint32_t);
> 
> To answer the question you asked in your cover letter, you cannot write
> this unless you can guarantee (((uintptr_t)index & 7) == 0) is true.
> Otherwise, this will produce a SIGBUS on SPARC, Alpha, MIPS, and some
> ARM systems, and it will perform poorly on PowerPC and other ARM
> systems[0].
> 
> If you got that pointer from malloc and have only indexed multiples of 8
> on it, you're good.  But if you're not sure, you probably want to use
> memcpy.  If the compiler can determine that it's not necessary, it will
> omit the copy and perform a direct load.

I think get_be32() does exactly what we want for the ewah_size read. For the last_update one, we don't have a get_be64() yet, but it should be easy to make based on the 16/32 versions.

(I note also that time_t is not necessarily 64-bits in the first place, but David said something about this not really being a time_t).

-Peff