From: Michael Haggerty <mhagger@alum.mit.edu>
To: David Kastrup <dak@gnu.org>, Andreas Schwab <schwab@linux-m68k.org>
Cc: Johannes Sixt <j6t@kdbg.org>,
"brian m. carlson" <sandals@crustytoothpaste.net>,
git@vger.kernel.org
Subject: Re: [PATCH 1/9] Define a structure for object IDs.
Date: Mon, 05 May 2014 12:52:28 +0200 [thread overview]
Message-ID: <53676D6C.4010009@alum.mit.edu> (raw)
In-Reply-To: <87d2fstuzw.fsf@fencepost.gnu.org>
On 05/05/2014 11:50 AM, David Kastrup wrote:
> The case we are talking about is basically passing a pointer to some
> actual bonafide toplevel unsigned char [20] object to a routine that
> expects a pointer to a struct _only_ containing one such
> unsigned char [20] element.
>
> This is the situation we have to deal with if a caller has not been
> converted to using such a struct, but the called function does.
If the rewrite is done by first changing data structures and then
changing functions in caller -> callee order then (1) the deltas can be
pretty small, and (2) such illegal casting should be unnecessary.
> More seriously, this is the situation we have to deal with when our SHA1
> is actually embedded in some header or whatever else that is actually
> available only inside of a larger byte buffer.
>
> In that case, the standard does not permit us converting the address
> where that SHA1 is into a pointer to struct. It may well be that this
> will fall under the "let's ignore the standard and write for "sensible"
> compilers/architectures" dictum, but if it doesn't, it might be
> necessary to first copy the data to a struct before passing it to
> routines expecting a pointer to struct.
This sounds dangerous even for a "sensible" compiler. For example, I
can imagine that a sensible compiler might make the assumption that a
sha1 field that it knows was obtained from oid->sha1 is word-aligned,
and generate optimized code based on that assumption, even though it
otherwise wouldn't have had trouble working with unaligned (unsigned
char *) pointers.
Michael
--
Michael Haggerty
mhagger@alum.mit.edu
http://softwareswirl.blogspot.com/
next prev parent reply other threads:[~2014-05-06 16:11 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-03 20:12 [RFC PATCH 0/9] Use a structure for object IDs brian m. carlson
2014-05-03 20:12 ` [PATCH 1/9] Define " brian m. carlson
2014-05-04 6:07 ` Michael Haggerty
2014-05-04 9:21 ` Johannes Sixt
2014-05-04 9:43 ` David Kastrup
2014-05-04 10:55 ` Andreas Schwab
2014-05-04 20:18 ` Johannes Sixt
2014-05-04 21:31 ` Andreas Schwab
2014-05-05 5:19 ` David Kastrup
2014-05-05 9:23 ` Andreas Schwab
2014-05-05 9:33 ` James Denholm
2014-05-05 9:50 ` David Kastrup
2014-05-05 10:52 ` Michael Haggerty [this message]
2014-05-05 11:05 ` Andreas Schwab
2014-05-05 11:23 ` David Kastrup
2014-05-05 18:16 ` Felipe Contreras
2014-05-04 12:29 ` Duy Nguyen
2014-05-04 16:07 ` brian m. carlson
2014-05-04 16:48 ` Andreas Schwab
2014-05-04 17:07 ` David Kastrup
2014-05-04 17:24 ` Andreas Schwab
2014-05-04 17:44 ` David Kastrup
2014-05-04 18:01 ` Andreas Schwab
2014-05-03 20:12 ` [PATCH 2/9] bisect.c: convert to use struct object_id brian m. carlson
2014-05-03 20:12 ` [PATCH 3/9] archive.c: " brian m. carlson
2014-05-03 20:12 ` [PATCH 4/9] zip: use GIT_OID_HEXSZ for trailers brian m. carlson
2014-05-03 20:12 ` [PATCH 5/9] branch.c: convert to use struct object_id brian m. carlson
2014-05-03 20:12 ` [PATCH 6/9] bulk-checkin.c: " brian m. carlson
2014-05-03 20:12 ` [PATCH 7/9] bundle.c: convert leaf functions to " brian m. carlson
2014-05-06 14:42 ` Michael Haggerty
2014-05-03 20:12 ` [PATCH 8/9] cache-tree: convert struct cache_tree to use object_id brian m. carlson
2014-05-06 14:53 ` Michael Haggerty
2014-05-06 15:02 ` Michael Haggerty
2014-05-03 20:12 ` [PATCH 9/9] diff: convert struct combine_diff_path to object_id brian m. carlson
2014-05-06 15:08 ` Michael Haggerty
2014-05-03 22:49 ` [RFC PATCH 0/9] Use a structure for object IDs brian m. carlson
2014-05-04 6:35 ` Michael Haggerty
2014-05-04 9:19 ` Johannes Sixt
2014-05-04 17:54 ` brian m. carlson
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