[cfe-dev] [LLVMdev] Clang devirtualization proposal

[Hal Finkel]

----- 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.

 -Hal

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