[swift-evolution] Pitch: Cross-module inlining and specialization

[Taylor Swift]

See the thread
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170731/038571.html>
from july over generic trig functions, where @_specialize() + @_inlineable
had a small but consistent performance penalty relative to @_inlineable
alone.

On Thu, Oct 5, 2017 at 1:32 AM, Slava Pestov <spestov at apple.com> wrote:

>
>
> On Oct 4, 2017, at 11:04 PM, Taylor Swift <kelvin13ma at gmail.com> wrote:
>
>
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> On Oct 5, 2017, at 12:52 AM, Slava Pestov <spestov at apple.com> wrote:
>
>
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> On Oct 4, 2017, at 9:40 PM, Taylor Swift via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> i’m just tryna follow along here && this is probably a dumb question, but
> is it possible for a generic function to be emitted as a set of specialized
> functions into the client, but not inlined everywhere? It can be the case
> where a large generic function gets slowed down by the large number of
> generic operations inside it but it doesn’t make sense for it to be inlined
> completely.
>
>
> This is already possible. The optimizer doesn’t have to inline an
> @_inlineable function at its call site; it can emit a call to a specialized
> version instead.
>
> Slava
>
>
> Is there a reason using @_specialize() and @_inlineable together is slower
> than using @_inlineable by itself?
>
>
> By specialization, I mean the optimizer pass which takes a function body
> and substitutes generic parameters with statically-known types.
>
> I’m not sure what your question means though. Adding a @_specialize
> attribute should never make anything slower. Rather it makes the optimizer
> eagerly emit specializations of a function in the defining module. You can
> think of @_specialize and @inlinable as mostly mutually exclusive; either
> you publish the complete function body for clients to optimize as they
> please, or you publish a fixed set of specializations.
>
> You might prefer the latter for secrecy (serialized SIL is much closer to
> source code than machine code), but the the former enables more general
> optimizations.
>
> Slava
>
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