[cfe-dev] Lambdas and the ABI?

[Hal Finkel via cfe-dev]

On 03/21/2017 08:45 AM, Hubert Tong wrote:
> This question was explored in an other context here:
> http://sourcerytools.com/pipermail/cxx-abi-dev/2013-January/002544.html

Very interesting, thanks! So the conclusion was that, " The ABI will 
need to specify the layout of closure types with weak linkage." In some 
cases (enumerated here: 
https://itanium-cxx-abi.github.io/cxx-abi/abi.html#closure-types) the 
layout must be fixed (because it must be consistent across translation 
units).

I imagine that size/alignment might also need to be fixed in cases where 
the source program explicitly depends on them (e.g. doing 
sizeof(lambda), new auto(lambda)).

  -Hal

>
> -- HT
>
> On Tue, Mar 21, 2017 at 9:03 AM, Hal Finkel via cfe-dev 
> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>
>     Hi Richard, Chandler, John, et al.,
>
>     "Quick" question: What aspects, if any, of a C++ lambda (e.g.
>     size, layout, alignment) leak into the ABI or are (potentially)
>     semantically visible?
>
>     Context...
>
>     We're investigating late lowering/outlining schemes to improve
>     code generation for OpenMP and other parallel programming models.
>     One important advantage of outlining late is that the IR-level
>     optimizer still has access to the pointer-aliasing and loop
>     information from the containing function. There are natural
>     benefits to C++ lambdas as well. Lambdas that are used "in place"
>     (i.e. only have one call site) should always be inlined, so the
>     issues don't come up there, but for lambdas that have multiple
>     call sites, or worse, escape (via std::function or some other
>     type-erasure mechanism), we can get suboptimal optimization
>     results for the body of the lambda. It would seem sad to fix this
>     for OpenMP but not for lambdas.
>
>     However, optimizing before outlining could mean changes in what
>     variables need to be captured (not semantically, but at the IR
>     level), and so I'd like to think through what would constrain the
>     optimizer's freedom to act in this regard.
>
>     John, I'm curious about how this might apply to Objective C blocks
>     as well.
>
>     Thanks again,
>
>     Hal
>
>     -- 
>     Hal Finkel
>     Lead, Compiler Technology and Programming Languages
>     Leadership Computing Facility
>     Argonne National Laboratory
>
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>

-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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