Re: [PATCH v1 2/5] Teach git to optionally utilize a file system monitor to speed up detecting new or changed files.
- Date: Mon, 15 May 2017 20:34:15 -0400
- From: Jeff King <peff@xxxxxxxx>
- Subject: Re: [PATCH v1 2/5] Teach git to optionally utilize a file system monitor to speed up detecting new or changed files.
On Tue, May 16, 2017 at 12:22:14AM +0000, brian m. carlson wrote:
> 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
> 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).