Thanks for your responses so far. It took me a while to fully appreciate what all this means.
I have read through John's mail thread on llvm-dev about adding the swiftcc convention to LLVM/Clang. I don't think that discussion mentioned adding Clang support for C++ functions returning aggregates in multiple registers for targets that currently use sret, because, in John's words, "we don't use the Swift CC to call C functions." But aren't these runtime functions using TwoWordPair::Return doing exactly that? On i386 and ARM, the TwoWordPair::Return type is masqueraded as a 64-bit unsigned long long so that the returned values are passed in a register pair. This trick is not portable, and infeasible on at least s390x. A portable solution would be to implement such runtime functions in Swift, and make them retrieve the needed results from two calls to C++, instead of one call to a C++ function that returns a pair. The Swift function can then return value pairs as usual. This seems much cleaner and simpler. Bryan rjmcc...@apple.com wrote on 2016-03-22 03:51:52 PM: > On Mar 22, 2016, at 12:25 PM, Joe Groff <jgr...@apple.com> wrote: > > On Mar 22, 2016, at 12:06 PM, Bryan Chan <bryan.c...@ca.ibm.com> wrote: > > > > > jgr...@apple.com wrote on 2016-03-22 02:37:47 PM: > > > > > > > The runtime functions are hacked in various unsavory ways to give > > > > them Swift-like calling conventions. Many of them also have > > > > important semantics that the compiler needs to be aware of, so if > > > > they aren't exported as standard library API in some way, you really > > > > shouldn't interact with them directly. > > > > (swift_class_getInstanceExtents happens to be OK, but what are you > > > > trying to do with it?) > > > > > > I am trying to build Swift on a different architecture (s390x). This > > > function was causing crashes due to the ABI mismatch, and I also > > > found that the issue is not 100% fixed on x86 either, so I wondered > > > if the community has an opinion on the right way to deal with this > > > in the runtime, or if the answer is simply "don't do this for > > > 3-scalar returns". > > > > Yeah, there's no good way right now to return three scalars from > > Clang. swift_class_getInstanceExtents and a few other runtime > > entrypoints like allocBox and allocError use the > > `TwoWordPair::Return` hack to use types that happen to return in two > > registers on i386, x86_64, armv7, and arm64. If s390x's C ABI > > doesn't define any such types, you might need to wait for proper > > support for Swift calling conventions in Clang. I believe it's on > > John McCall's near-term todo list to do that. > > To be clear, you'll still need to implement the swiftcc convention > for your target in LLVM/clang, so it will still take some compiler > implementation expertise to get right. But at least the > compatibility assumptions will be explicit.
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