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Re: As-if Infinitely Ranged Integer Model
- From: "Joseph S. Myers" <joseph at codesourcery dot com>
- To: Robert Seacord <rcs at cert dot org>
- Cc: "'gcc at gcc dot gnu dot org'" <gcc at gcc dot gnu dot org>
- Date: Fri, 24 Jul 2009 11:51:48 +0000 (UTC)
- Subject: Re: As-if Infinitely Ranged Integer Model
- References: <6A6ECBC0FF056D4998DC6232D489ED6A0155F9710EE7@EXCHANGE.sei.cmu.edu> <C10B9E3687C68B4D98F6ADFD3C3CDDB701562FC49E31@EXCHANGE.sei.cmu.edu>
As further comments, it's unfortunate that this document has chosen
certain badly thought out basis documents and concepts.
On page 13 you have a short table of "critical undefined behavior" and an
attempt to define "bounded undefined behavior", that ignore all my
previous comments in this area, especially the detailed list of cases I
sent in reflector message 11499. As I said there:
First, there isn't a clear definition of what the bounds are; if you are
trying to bound possible behavior, I think it needs to be clear enough to
define a set of possible executions in the abstract machine.
If people want predictability across implementations, they must stop
ignoring the issues and hoping that "undefined but not quite so undefined
as all that" will magically solve their problems, and instead seriously
put in the weeks or months of work in detailed analysis of every instance
of undefined behavior and proper definitions in the abstract machine (that
are actually implementable) to bound the possible executions; removing
"undefined" does not introduce semantics that were never previously needed
or agreed, only careful thought and drafting and textual review work in
hundreds of individual cases can do so. The definition in your document
fully permits bounded undefined behavior to make daemons fly out of your
nose, as that is not an out-of-bounds store.
Then you are building on the runtime-constraint mechanism and rsize_t of
TR 24731-1. TR 24731-1 is considered useless in the Linux world, and not
implemented in the GNU C Library, and with good reason; see
<http://sourceware.org/ml/libc-alpha/2007-09/msg00069.html>. If you want
general adoption in the Linux world you will need to extract those pieces
from the pile of useless, duplicative and prior-art-ignoring functions
that is most of TR 24731-1 and demonstrate that despite their background
the runtime-constraints and rsize_t have more general utility.
Joseph S. Myers