[cfe-dev] Smart Pointer Lifetime Optimizations

[Zoe Carver via cfe-dev]

John,

Thanks, those are good points. I think we can still remove one of the
destructors (which could also be done by a more powerful DSE+load
propagation) but, you're right; one needs to stay.

> This can only be optimized with a more global, interprocedural
optimization that shifts responsibility to owner to destroy its argument.

I'll think about implementing something like this, but I suspect any
possible optimizations will already happen with inlining and analysis.

Thanks for the response,
Zoe

On Fri, Jun 5, 2020 at 1:09 PM John McCall <rjmccall at apple.com> wrote:

> On 5 Jun 2020, at 14:45, Zoe Carver via cfe-dev wrote:
>
> Hello all,
>
>
> I'm planning to do some work to add lifetime optimization passes for smart
> pointers and reference-counted objects. I'll use this email as a sort of
> proposal for what I hope to do.
>
>
> *Scope*
>
>
> As I'm developing the pass, I'm trying to keep it general and create
> utilities that could work across multiple smart pointers. But, right now,
> I'm focussing on unique_ptr and applying specific ownership optimizations
> to
> unique_ptr only.
>
>
> *unique_ptr Optimzations*
>
>
> The pass I'm currently developing adds a single, simple, optimization:
> constant fold the destructor based on ownership information. unique_ptr has
> a lot of ownership information communicated with reference semantics. When
> a
> unique_ptr is moved into another function, that function takes over
> ownership of the unique_ptr, and subsequent destructors can be eliminated
> (because they will be no-ops). Otherwise, branchless functions are often
> complicated after inlining unique_ptr's destructor so, this optimization
> should be fairly beneficial.
>
>
> unique_ptr's reset and release methods both complicate this optimization a
> bit. Because they are also able to transfer and remove ownership, all
> unknown instructions must be ignored. However, in the future, knowledge of
> those methods might be able to make the pass more robust.
>
>
> With unique_ptr, it's difficult to prove liveness. So, it is hard to
> constant fold the destructor call to always be there. Maybe in the future,
> this would be possible, though (with sufficient analysis).
>
>
> Last, an optimization that I hope to do is lowering the unique_ptr to a raw
> pointer if all lifetime paths are known. I think removing this layer of
> abstraction would make it easier for other optimization passes to be
> successful. Eventually, we may even be able to specialize functions that
> used to take a unique_ptr to now take a raw pointer, if the argument's
> lifetime was also able to be fully analyzed.
>
>
> *Lifetime Annotations*
>
>
> Right now, the pass relies on (pre-inlined) function calls to generate
> ownership information. Another approach would be to add ownership
> annotations, such as the lifetime intrinsics (i.e. llvm.lifetime.start).
>
>
> *ARC Optimizations*
>
>
> There are a huge number of large and small ARC optimizations already in
> LLVM. For unique_ptr specifically, I'm not sure these are of any benefit
> because unique_ptr doesn't actually do any reference counting. But, later
> on, when I start working on generalizing this pass to support more smart
> pointers (specifically shared_ptr) I think the ARC optimization pass, and
> especially the utilities it contains, could be very beneficial. If anyone
> has experience with ARC optimizations, I'd love to hear your thoughts on
> extending them to other reference counted objects.
>
>
> *trivial_abi and Hidden References*
>
>
> Arthur O'Dwyer made a good point, which is that a lot of these
> optimizations can be applied when with the trivial_abi attribute. However,
> given that's not a standard attribute and these optimizations only *happen*
> to work with trivial_abi (i.e., in a more complicated program, they may not
> continue to work). I think lifetime utilities and specific lifetime
> optimization passes are still beneficial (especially if they can be applied
> to other smart pointers in the future).
>
>
> Because all smart pointers have non-trivial destructors, they are always
> passed by hidden references. With unique_ptr, this is as simple as
> bit-casting the pointer member to unique_ptr, which would allow for it to
> be lowered to a single raw pointer instead of a stack-allocated object.
> Even without the trival_abi attribute, I think this is an optimization that
> could be done.
>
>
> *Results*
>
>
> Here's the unique_ptr pass I've been talking about: ⚙ D81288 Opt Smart
> pointer lifetime optimizations pass. <https://reviews.llvm.org/D81288>
>
> For reference, here are the before and after results:
>
> Clang trunk (four branches): Compiler Explorer
> <https://godbolt.org/z/bsJFty>
>
> With optimizations (branchless): https://pastebin.com/raw/mQ2r6pru
>
> Unfortunately, these are not legal optimizations for your test case:
>
>    -
>
>    guaranteed is permitted to escape a reference (or pointer) to the
>    object it was passed. Tat references and pointers remain valid
>    until the object goes out of scope.
>    -
>
>    The object can be mutated through that reference because the underlying
>    object is not const. Being passed a const reference is not a
>    semantic contract in C++.
>    -
>
>    Through a combination of the above, the call to owner may change
>    the value of p, and so the caller may not rely on it still being
>    in a trivially-destructible state after that call.
>    -
>
>    owner may leave the value of its parameter object in a
>    non-trivially-destructible state, and under the Itanium C++ ABI,
>    cleaning
>    up that object is the caller’s responsibility. I agree that this is a
>    bad rule for optimization purposes, but it’s the rule. This can only be
>    optimized with a more global, interprocedural optimization that shifts
>    responsibility to owner to destroy its argument.
>
> John.
>
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