[llvm-dev] [PATCH] D12923: Add support for function attribute "notail"

[Philip Reames via llvm-dev]

On 09/29/2015 11:38 AM, Akira Hatanaka wrote:
> On Tue, Sep 29, 2015 at 10:49 AM, Philip Reames 
> <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote:
>
>
>
>     On 09/28/2015 10:38 PM, Sanjoy Das wrote:
>
>
>         > That was what I had in mind: the function attribute blocks
>         tail call for statically direct calls but doesn't promise
>         > anything (in fact, does nothing) for indirect calls.
>         >
>         > Do you think we shouldn't make any promises for statically
>         direct calls either? I don't see why it's hard to keep the
>         > promise that direct tail calls will be blocked. Do you have
>         a particular optimization in mind that would be difficult or
>         > impossible to implement if we promised to block direct calls?
>
>         I think in this scheme we'll have problems around
>         devirtualization.
>         For instance, you could start with a indirect call to a notail
>         target
>         that would get TCO'ed (as an indirect call), after which
>         devirtualization or load/store motion would turn the indirect call
>         into a direct call; and you'd end up with a direct tail call to a
>         notail target.
>
>     Sanjoy hits on some of the same concerns I had with this. In
>     particular, there's a distinction between what the language
>     specification defines as a direct call and what the
>     optimizer/compiler might turn into a direct call.  At the source
>     layer, we can *only* make guarantees about the former.  Any
>     specification we write has to account for the fact the later are
>     an implementation detail entirely under the control of the
>     compiler. i.e. optimizations can not break the semantics.  You
>     have to ensure this when defining the semantics.
>
>
> By "statically direct calls", I meant the set of calls that can be 
> determined to be direct at the source level, which is normally a 
> subset of the calls that end up being direct calls after all the 
> optimization passes are run. I agree that we can probably promise or 
> guarantee that tail call will be blocked for direct calls at the 
> source level. We don't guarantee anything for indirect calls at the 
> source level.
>
>
>
>         I don't know what your requirements are, but if I were you I'd
>         design
>         the `notail` marker to be logically part of a function's calling
>         convention.  That way functions marked `notail` get to do
>         things that
>         are legal only if the call to them wasn't a tail call, because all
>         well-defined calls constrain the caller to match callee's
>         expectation
>         with respect to tail calls.  In the CFE, you could make the
>         `notail`
>         attribute have the same restrictions / conventions as things like
>         `fastcall`.
>
>     I disagree with this slightly.  As I've stated previously, I think
>     there are two distinct semantics here: a) this call must be a
>     notail call, and b) we'd "like" this to be a no tail, but it's
>     optional. We need to not mix them.
>
>
>
> The semantics of the tail call marker I'm looking for is close to b), 
> but differs in whether we say it's optional or we guarantee that we 
> block tail call for a certain subset of calls (direct calls at the 
> source level). I'm inclined to have the tail call marker mean "notail 
> is guaranteed if the call is direct at the source level", but I wonder 
> if it'll come back to bite us.
I think this is probably fine, but you need someone with clang/C++ spec 
knowledge that I don't have to help with the definition.
>
> Also, this tail call marker won't be used to enable optimizations on 
> the marked function that would be available only if it were known the 
> call to the function isn't TCO'ed. That might be an interesting idea 
> to other people, but it's not something that is needed for our use case.
>
>         -- Sanjoy
>
>
>

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