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Re: C as used/implemented in practice: analysis of responses
- From: Peter Sewell <Peter dot Sewell at cl dot cam dot ac dot uk>
- To: Joseph Myers <joseph at codesourcery dot com>
- Cc: "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>, Kayvan Memarian <Kayvan dot Memarian at cl dot cam dot ac dot uk>
- Date: Sat, 27 Jun 2015 00:48:15 +0100
- Subject: Re: C as used/implemented in practice: analysis of responses
- Authentication-results: sourceware.org; auth=none
- References: <CAHWkzRQ4+-s6RJeAnCHFDqfQDNaPDATRYkz73xvpwxqP4O9JjA at mail dot gmail dot com> <alpine dot DEB dot 2 dot 10 dot 1506261702360 dot 7819 at digraph dot polyomino dot org dot uk> <CAHWkzRTQndb7q152fPghp5JHzmQJ+2vesSD88ArpSnaAUMWXOw at mail dot gmail dot com> <alpine dot DEB dot 2 dot 10 dot 1506261923150 dot 28580 at digraph dot polyomino dot org dot uk>
- Reply-to: Peter dot Sewell at cl dot cam dot ac dot uk
On 26 June 2015 at 20:27, Joseph Myers <email@example.com> wrote:
> On Fri, 26 Jun 2015, Peter Sewell wrote:
>> > It's s simple matter of points-to analysis. &foo + anything may be
>> > assumed (in practice) to point to something within foo (or just past the
>> > end) and not to alias anything accessed through a pointer based on &bar.
>> > If the compiler can see something like &foo + (&bar - &foo) there is no
>> > guarantee of whether it will assume it to point within foo or bar and that
>> > may not be consistent for different uses (so it may end up concluding the
>> > pointer compares unequal to itself).
>> Ok, that's fine in some (perhaps most) situations, but it's not
>> compatible with what seems to be a significant body of systems code
>> out there - people mentioned important usages in FreeBSD, Linux, QEMU,
>> and other places. How can these be reconciled? We imagine:
> Does that code actually access the same object via both routes in code
> that might get moved past each other (or values get reused because the
> compiler didn't think they could have changed) as a consequence of the
> points-to analysis? If the aliasing isn't visible, it's less likely to
> cause problems.
It's a good question - quite possibly not, though it's hard to
investigate; really we have no idea. But I'm not sure how we could
state a condition like that precisely in some reasonable way, and if
the conditions under which this is safe become too complex, that in
itself becomes a failing in the language definition - at the end of
the day, it has to be comprehensible.
More generally, the programming-language design tradeoff between
simplicity on the one hand and performance and expressivity on the
other is always a difficult thing to manage. The wide range of
opinions out there about what C behaviour can be relied on suggests
that C may have veered too far to the latter in some respects.
>> a) Compilation of that systems code could turn off this analysis (and
>> whatever optimisation depends on it) entirely. What's the cheapest
>> way to do that?
> I think -fno-tree-pta should disable it (I don't know what the cost of
> that is).
> Joseph S. Myers