> On Oct 13, 2016, at 10:46 AM, John McCall via swift-dev <swift-dev@swift.org> > wrote: > >> On Oct 13, 2016, at 9:04 AM, Joe Groff via swift-dev <swift-dev@swift.org >> <mailto:swift-dev@swift.org>> wrote: >> >>> On Mar 1, 2016, at 1:33 PM, Joe Groff via swift-dev <swift-dev@swift.org >>> <mailto:swift-dev@swift.org>> wrote: >>> >>> In swift_retain/release, we have an early-exit check to pass through a nil >>> pointer. Since we're already burning branch, I'm thinking we could pass >>> through not only zero but negative pointer values too on 64-bit systems, >>> since negative pointers are never valid userspace pointers on our 64-bit >>> targets. This would give us room for tagged-pointer-like optimizations, for >>> instance to avoid allocations for tiny closure contexts. >> >> I'd like to resurrect this thread as we look to locking down the ABI. There >> were portability concerns about doing this unilaterally for all 64-bit >> targets, but AFAICT it should be safe for x86-64 and Apple AArch64 targets. >> The x86-64 ABI limits the userland address space, per section 3.3.2: >> >> Although the AMD64 architecture uses 64-bit pointers, implementations are >> only required to handle 48-bit addresses. Therefore, conforming processes >> may only use addresses from 0x00000000 00000000 to 0x00007fff ffffffff. >> >> Apple's ARM64 platforms always enable the top-byte-ignore architectural >> feature, restricting the available address space to the low 56 bits of the >> full 64-bit address space in practice. Therefore, "negative" values should >> never be valid user-space references to Swift-refcountable objects. Taking >> advantage of this fact would enable us to optimize small closure contexts, >> Error objects, and, if we move to a reference-counted COW model for >> existentials, small `Any` values, which need to be refcountable for ABI >> reasons but don't semantically promise a unique identity like class >> instances do. > > This makes sense to me. if (x <= 0) return; should be just as cheap as is (x > == 0) return;
Conversely, I wanted to try to remove such nil checks. Currently they look haphazard: some functions have them and some do not. Allowing ABI space for tagged pointer objects is a much bigger problem than the check in swift_retain/release. For example, all vtable and witness table dispatch sites to AnyObject or any other type that might someday have a tagged pointer subclass would need to compile in a fallback path now. You can't dereference a tagged pointer to get its class pointer. -- Greg Parker gpar...@apple.com <mailto:gpar...@apple.com> Runtime Wrangler
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