[llvm-dev] [RFC] NewGVN

Arnaud De Grandmaison via llvm-dev llvm-dev at lists.llvm.org
Fri Nov 18 01:42:48 PST 2016


Slides and video available at https://archive.fosdem.org/2015/schedule/event/the_cheri_cpu/

Kind regards,

> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
> Davide Italiano via llvm-dev
> Sent: 18 November 2016 03:55
> To: David Chisnall
> Cc: llvm-dev
> Subject: Re: [llvm-dev] [RFC] NewGVN
> On Thu, Nov 17, 2016 at 12:31 AM, David Chisnall
> <David.Chisnall at cl.cam.ac.uk> wrote:
> > On 16 Nov 2016, at 21:56, Daniel Berlin <dberlin at dberlin.org> wrote:
> >>
> >> You keep talking about platforms, but llvm ir itself is not platform
> dependent.
> >> Can you give a reference in the language reference that says that this is
> not legal?
> >
> > Nothing in the LangRef (apart from the note about non-integral pointers,
> which was added recently) makes any claim about the representation of
> pointers.  Pointers in LLVM IR have always been opaque and must explicitly
> be bitcast or inttoptr / ptrtoint cast to be used as if they were integers.
> >
> > We have had discussions on the list previously about tightening up the
> semantics of inttoptr and ptrtoint.
> >
> >> IE what loads do *on your platform* is completely irrelevant to whether
> the IR code is legal or not, only what it codegens to.
> >>
> >> LLVM's type semantics (and pointers may not have types, but the load
> operations produce values that do) are also not defined in terms of platform,
> but in terms of what datalayout says, etc.
> >
> > GVN is materialising loads that go beyond the bounds of an object.  This is
> undefined behaviour in C and there is nothing in the LangRef that indicates
> that this should be valid.  It is only potentially valid because, on platforms
> with a page-based MMU as the sole form of memory protection, if you only
> round up to a power of two then you will still be in the same page (and,
> likely, cache line) so you will get some unspecified data and can ignore it.
> >
> >> What you want seems to be non-integral pointer types.
> >>
> >> Which are experimental:
> >> "LLVM IR optionally allows the frontend to denote pointers in certain
> address spaces as “non-integral” via the datalayout string. Non-integral
> pointer types represent pointers that have an unspecified bitwise
> representation; that is, the integral representation may be target dependent
> or unstable (not backed by a fixed integer).
> >> inttoptr instructions converting integers to non-integral pointer types are
> ill-typed, and so are ptrtoint instructions converting values of non-integral
> pointer types to integers. Vector versions of said instructions are ill-typed as
> well."
> >>
> >> One of the reasons it's experimental is because nobody has made it work
> in all cases.
> >> I think whoever wants this to work is going to have to drive fixing it and
> making it work sanely.
> >
> > Actually, that isn’t what I want, because we do define inttoptr and ptrtoint
> for our architecture.  You can’t implement C without them (or some
> equivalent) working and we have a fully working C / Objective-C compiler
> (C++ in progress) using LLVM.  ptrtoint is always valid for us, inttoptr may give
> null depending on the ABI and environment.
> >
> > I gave a talk in the LLVM track at FOSDEM a couple of years ago about the
> things that are needed to make LLVM work correctly for targets where
> integers are not pointers.  We have done most of this work, but it is not
> helped by people propagating the ‘integers are pointers’ assumption (which
> the LangRef has always been *very* careful not to state) in passes.
> >
> Do you happen to have a link for the talk? We'll try to make sure this works in
> the new pass.
> --
> Davide
> "There are no solved problems; there are only problems that are more or
> less solved" -- Henri Poincare
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