> On Mar 17, 2017, at 4:07 PM, Joe Groff <jgr...@apple.com> wrote: > >> On Mar 17, 2017, at 4:04 PM, Greg Parker <gpar...@apple.com >> <mailto:gpar...@apple.com>> wrote: >> >>> On Mar 17, 2017, at 8:19 AM, Joe Groff <jgr...@apple.com >>> <mailto:jgr...@apple.com>> wrote: >>> >>> It might at some point, but I was thinking of using this in the runtime for >>> now. We already have a few statically-emitted objects for the empty >>> containers that could benefit from this, but they could presumably use API >>> from RefCount.h to grab a constexpr bit pattern with which to initialize >>> themselves. >> >> Good: we already support that usage. Struct HeapObject has a constexpr >> initializer that sets the refcounts to that value. See >> stdlib/public/stubs/GlobalObjects.cpp for some similar uses. >> >> (But wait, objects for empty containers are already in GlobalObjects.cpp. >> Are you adding more such objects?) > > I was considering it as a possibility for statically emitting literal key > path objects as part of Foundation's recent language proposal > (https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170313/033998.html > > <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170313/033998.html>). > Even for this case, I don't think I'd have the compiler statically emit the > bit pattern, but have accesses to the object go through a runtime call that > lazily reifies it. (That way, if we change the object format, we can also > still reallocate the older compiler-emitted format.)
The stable ABI will have affordances for statically-emitted objects. For now you can use the machinery in GlobalObjects.cpp. When the stable ABI progresses we'll find all uses of that code and update them. One likely possibility is that we emit some bit pattern directly, but also emit enough metadata so that the runtime could find all such objects at launch time. (For example on Mach-O the objects could all be emitted into the same section.) That way we get high performance now but preserve the ability to rewrite the objects in the future, on the assumption that such ABI evolution will be rare. The Objective-C runtime uses this technique in several places. -- Greg Parker gpar...@apple.com <mailto:gpar...@apple.com> Runtime Wrangler
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