[cfe-dev] [LLVMdev] Clang devirtualization proposal

[Chandler Carruth]

On Sat, Aug 1, 2015 at 6:39 AM Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> > From: "Sanjoy Das" <sanjoy at playingwithpointers.com>
> > To: "Reid Kleckner" <rnk at google.com>
> > Cc: "Piotr Padlewski" <prazek at google.com>, "cfe-dev at cs.uiuc.edu
> Developers" <cfe-dev at cs.uiuc.edu>, "LLVM Developers
> > Mailing List" <llvmdev at cs.uiuc.edu>
> > Sent: Saturday, August 1, 2015 1:22:50 AM
> > Subject: Re: [LLVMdev] [cfe-dev] Clang devirtualization proposal
> >
> > On Fri, Jul 31, 2015 at 6:18 PM, Reid Kleckner <rnk at google.com>
> > wrote:
> > > Consider this pseudo-IR and some possible transforms that I would
> > > expect to
> > > be semantics preserving:
> > >
> > > void f(i32* readonly %a, i32* %b) {
> > >   llvm.assume(%a == %b)
> > >   store i32 42, i32* %b
> > > }
> > >   ...
> > >   %p = alloca i32
> > >   store i32 13, i32* %p
> > >   call f(i32* readonly %p, i32* %p)
> > >   %r = load i32, i32* %p
> > >
> > > ; Propagate llvm.assume info
> > > void f(i32* readonly %a, i32* %b) {
> > >   store i32 42, i32* %a
> > > }
> > >   ...
> > >   %p = alloca i32
> > >   store i32 13, i32* %p
> > >   call f(i32* readonly %p, i32* %p)
> > >   %r = load i32, i32* %p
> >
> > I'd say this first transformation is incorrect.  `readonly` is
> > effectively part of `%a`'s "type" as it constrains and affects the
> > operations you can do on `%a`. Even if `%b` is bitwise equivalent to
> > `%a` at runtime, it is "type incompatible" to replace `%a` with `%b`.
> >
> > This is similar to how you cannot replace `store i32 42, i32
> > addrspace(1)* %a` with `store i32 42, i32 addrspace(2)* %b`, even if
> > you can prove `ptrtoint %a` == `ptrtoint %b` -- the nature of `store`
> > is dependent on the type of the pointer you store through.
> >
> > The glitch in LLVM IR right now is that the `readonly`ness of `%a` is
> > not modeled in the type system, when I think it should be. An `i32
> > readonly*` should be a different type from `i32*`.  In practice this
> > may be non-trivial to get right (for instance `phi`s and `selects`
> > will either have to do a type merge, or we'd have to have explicit
> > type operators at the IR level).
>
> We could do this, but then we'd need to promote these things to
> first-class parts of the type system (and I'd need to put further thought
> about how this interacts with dynamically-true properties at callsites and
> inlining).
>
> The alternative way of looking at it, which is true today, is that
> @llvm.assume is not removed even when its information is 'used'. It
> appears, given this example, that this is actually required for
> correctness, and that dead-argument elimination needs to specifically not
> ignore effectively-ephemeral values/arguments.
>

What follows is an off-the-cuff response. I'm still thinking through it,
but wanted to let others do so as well.

There is yet another interpretation that we might use: the final
transformation Reid proposed is actually correct and allowed according to
the IR.

Specifically, we could make 'readonly' a property of the memory much like
aliasing is. That would mean that the function promises not to modify the
memory pointed to by %a in this example. The optimizer gets to ignore any
such modifications while remaining correct.

This would, in turn, mean that the store in Reid's "@f" function is an
unobservable in the case that %a == %b through some dynamic property,
whatever that may be. And as a consequence, the store-forwarding is a
correct transformation.

-Chandler
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