[swift-dev] Having 64-bit swift_retain/release ignore all negative pointer values
gparker at apple.com
Thu Oct 13 15:18:18 CDT 2016
> On Oct 13, 2016, at 10:46 AM, John McCall via swift-dev <swift-dev at swift.org> wrote:
>> On Oct 13, 2016, at 9:04 AM, Joe Groff via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
>>> On Mar 1, 2016, at 1:33 PM, Joe Groff via swift-dev <swift-dev at swift.org <mailto:swift-dev at 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 gparker at apple.com <mailto:gparker at apple.com> Runtime Wrangler
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