[cfe-dev] Smart Pointer Lifetime Optimizations

Sun Jun 7 15:52:34 PDT 2020

John, sorry for my delayed response.

*> Can you explain in more detail which destructor you think you can
eliminate?*

In the Godbolt link in my original post, there are two unique_ptrs, %3 and %
4. They are both passed to the move constructor (as the source and
destination). In the move constructor, the source's held pointer is nulled
out. If the next use of the pointer is the destructor (as is the case in
the Godbolt example) then, the destructor will be a no-op. In the optimized
code, it's a bit more difficult to see. But, the dead store is still there.
Two instructions before the call to owner, null is stored into %8. Then, in
the 3rd block (%17) that same pointer (%8) is loaded and compared against
null (which is always true).

*> One idea I’ve personally been kicking around is some way to mark
declarations as having an “unstable ABI”...*

This is a super interesting idea. When I become more familiar with clang's
internals, I'd be interested in helping to implement it.

*> Probably more importantly, though, we could allow unstable-ness to be
detected with a type trait, and that would allow the standard library to
take advantage of it. *

We could actually do this for trivial_abi types too. If we added a builtin
type trait to check if a type has the trivial_abi attribute, libc++ could
conditionally give unique_ptr the trivial_abi attribute if its base type
also had the attribute. Additionally, we could add a config macro that
would do this globally when libc++ is in unstable ABI mode.

Best,

Zoe

On Sat, Jun 6, 2020 at 2:07 PM John McCall <rjmccall at apple.com> wrote:

> On 6 Jun 2020, at 13:47, Zoe Carver wrote:
>
> 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.
>
> Can you explain in more detail which destructor you think you can
> eliminate?
>
> 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.
>
> Yeah. For the narrow case of std::unique_ptr, since its operations
> are easily inlined and can be easily optimized after copy propagation,
> there’s not much more that can be done at a high level.
>
> Note that trivial_abi (if it could be adopted on std::unique_ptr)
> also changes the ABI to make the callee responsible for destruction.
> So as part of getting a more efficient low-level ABI, you also get a
> more optimizable high-level one.
>
> One idea I’ve personally been kicking around is some way to mark
> declarations as having an “unstable ABI”: basically, a guarantee that
> all the code that uses them will be compiled with a single toolchain,
> and therefore a license for the implementation to alter the ABI however
> it likes with any code that uses any of those declarations.
>
> A type would be unstable if it was composed even partially from a
> declaration marked unstable. So class Unstable would be unstable,
> but so would const Unstable * — and, crucially, so would
> std::unique_ptr<Unstable>. But for soundness reasons, this would
> need to ignore type sugar (so no marking typedefs), and it wouldn’t
> be able to automatically descend into fields.
>
> There are a few ways that we could use that directly in the compiler.
> The big restriction is that you’re not breaking ABI globally and so
> you always need an unstable “contaminant” that permits using the
> unstable ABI. For example, we can’t just change function ABIs
> for all unstable functions because function pointers have to remain
> compatible. On the other hand, programs aren’t allowed to call
> function pointers under the wrong type, so if the function type is
> unstable, we can change anything we want about its ABI.
>
> (For functions specifically, there’s another option: we could emit
> the functions with an unstable ABI and then introduce thunks that
> adapt the calling convention when the address is taken. But that’s
> a non-trivial code-size hit that we might have to do unconditionally.
> It also can’t adapt a callee-destroy ABI into a caller-destroy one
> without introducing an extra move, which isn’t necessarily semantically
> allowed.)
>
> Probably more importantly, though, we could allow unstable-ness to
> be detected with a type trait, and that would allow the standard
> library to take advantage of it. So std::unique_ptr<int> would
> be stuck with the stable ABI, but std::unique_ptr<Unstable> could
> switch to be trivial_abi.
>
> That does leave the problem of actually doing the annotation.
> Adding an attribute to every class is probably beyond what people
> would accept. There are several ways to do mass annotation. Pragmas
> are problematic because you don’t want to accidentally leave the
> pragma on when you exit a file and then have it cover a system
> include. We do have some pragmas that prevent file changes while
> the pragma is active, which is a decent solution for that problem.
> An alternative is to mark namespaces. That probably needs to be
> lexical: that is, you wouldn’t be able to mark the entire clang
> namespace, you would mark a specific namespace clang declaration
> in a single header. But that’s still much more manageable, and
> after all, the cost to missing an annotation is just a missed
> optimization.
>
> We could also implicitly make all anonymous-namespace declarations
> unstable.
>
> John.
>
> 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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