[llvm-dev] [RFC] NewGVN

[Daniel Berlin via llvm-dev]

On Wed, Nov 16, 2016 at 11:11 AM, David Chisnall <
David.Chisnall at cl.cam.ac.uk> wrote:

> On 16 Nov 2016, at 19:03, Davide Italiano <davide at FreeBSD.org> wrote:
> >
> >>>
> >>> For our target, this is only sound if you can show that the pointer was
> >>> used to read all of the bytes that you are loading (we have
> byte-granularity
> >>> memory safety).  Old GVN has no hooks for targets to specify whether
> this is
> >>> safe and so is implicitly assuming a page-based MMU.  This
> optimisation is
> >>> also unsound for M-profile ARM cores, though will fail occasionally
> there,
> >>> whereas it fails deterministically for us.
> >>>
> >>> Does it make any assumptions about the layout of memory in pointers?
> >>
> >
> > Which assumptions are you thinking of?
>
> My last merge from upstream was about a year ago (and a new one is long
> overdue), but there were issues where GVN was assuming that if it did a
> load of a pointer then a ptrtoint, then a truncation, that it would get the
> same result as doing a narrower load.  This is not the case in any platform
> where pointers are not simply integers (i.e. where you actually need
> inttoptr / ptrtoint instead of bitcast).
>

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?

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.

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
<http://llvm.org/docs/LangRef.html#langref-datalayout>. 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.



> David
>
>
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