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Re: C as used/implemented in practice: analysis of responses
- From: Joseph Myers <joseph at codesourcery dot com>
- To: Peter Sewell <Peter dot Sewell at cl dot cam dot ac dot uk>
- 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: Fri, 26 Jun 2015 19:27:56 +0000
- 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>
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
> 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
Joseph S. Myers