[swift-lldb-dev] Support for the Swift calling convention in lldb

[Jim Ingham]

I commented in the Radar.  

"step-out" captures the return and error values.  That's done in some hand-rolled code in lldb, so that would have to be modified.  Longer term it would be great if Swift (and Clang) had a function that did: Function Decl -> dwarf expression for return location, then lldb could just run that expression to pick up the return value.  But that's certainly not going to happen for the Empire release, so we'll just have to adjust the lldb code that grubs around for this information.

We do error returns at present because we know that there's a faux variable that holds the error address, and the DWARF describes where that is.  Provided that keeps working by some magic that Adrian will perform, then we won't need to do anything on the lldb side.

Jim

> On Oct 13, 2016, at 1:07 PM, Todd Fiala via swift-lldb-dev <swift-lldb-dev at swift.org> wrote:
> 
>> 
>> On Oct 13, 2016, at 12:54 PM, ematejska at apple.com wrote:
>> 
>> + Michael to see this thread (he’s the ABI DRI from the frontend side.)
>> 
>> Ewa
>>> On Oct 13, 2016, at 12:47 PM, Todd Fiala via swift-lldb-dev <swift-lldb-dev at swift.org> wrote:
>>> 
>>> Hi Arnold!
>>> 
>>> Thanks for the heads up.  Comments below.
>>> 
>>>> On Oct 13, 2016, at 12:32 PM, Arnold Schwaighofer via swift-lldb-dev <swift-lldb-dev at swift.org> wrote:
>>>> 
>>>> Hi LLDB team,
>>>> 
>>>> as part of the ABI work for this year we would like to adopt the swift calling convention.
>>>> 
>>>> I am working on the swift/llvm side of this.
>>>> 
>>>> <rdar://problem/19978563> Adopt the new Swift calling convention
>>>> 
>>>> The swift calling convention “swiftcc” together with the “swifterror” and “swiftself” attribute will change how many registers this calling convention will use for passing arguments and returning values. Furthermore, the “swifterror” and “swiftself” attribute will cause llvm to put the parameter marked with this attribute into a specific register at the call side.
>>>> 
>>>> The swift compiler will use this convention for native swift functions. We have to be able to call native swift functions from the runtime so there is also support in clang to define/declare functions with this calling convention in C/C++.
>>> 
>>> With Swift, we have a convention that we require LLDB and Swift versions to be identical in order to debug Swift code.  In the case of C/C++ code, we have no such lock-step requirements.  How would we identify C/C++ code that was using this calling convention?
>>> 
> 
> I’m not entirely sure of all the places that care.
> 
> * Possibly the unwinder, although that might not care since it needs to handle hand-rolled assembly and everything in between.  Jason could  say more here.
> 
> * Maybe the expression parser, if calling into methods with a new ABI?  Sean could probably say more here.
> 
>>>> 
>>>> This might have implications on the debugger.
>>>> 
>>>> My current plan is to finish the swift/llvm side of this work in the next couple weeks. There is a prototype at https://github.com/aschwaighofer/swift/tree/native_calling_convention_wip that can be tried out today.
>>>> 
>>>> There are two radars that track work related to lldb and dwarf support:
>>>> 
>>>> <rdar://problem/24489517> DWARF support for the new Swift calling convention
>>>> <rdar://problem/25471028> LLDB support for new Swift calling convention
>>>> 
>>>> 
>>>> Best,
>>>> Arnold
>>>> 
>>>> 
>>>> 
>>>> 
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