[cfe-dev] [LLVMdev] Clang devirtualization proposal

[Hal Finkel]

----- Original Message -----
> From: "Chandler Carruth" <chandlerc at google.com>
> To: "Hal Finkel" <hfinkel at anl.gov>, "Sanjoy Das" <sanjoy at playingwithpointers.com>
> Cc: "cfe-dev at cs.uiuc.edu Developers" <cfe-dev at cs.uiuc.edu>, "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Friday, August 7, 2015 5:52:04 PM
> Subject: Re: [cfe-dev] [LLVMdev] Clang devirtualization proposal
> 
> 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.

We could certainly do this, but it will obviously make inference harder. That might not be a good thing.

As I said earlier, the original problem highlighted by Reid's example cannot currently occur: that could only happen if you remove the @llvm.assume call (thus making the other argument unused). We already don't do this (because the assumes could be useful if later inlined), and now we have a second reason. Regardless, because we don't actively remove @llvm.assume, I'm not convinced the current state of things is currently broken.

 -Hal

> 
> 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
> 
> 
> 
> 
> 
> -Hal
> 
> > 
> > -- Sanjoy
> > _______________________________________________
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> > 
> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory