[stable/20210726] Swift async extended frame info (LLVM only) #3253
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…oying When back-deploying Swift async code we can't always toggle the flag showing an extended frame is present because it will confuse unwinders on systems released before this feature. So in cases where the code might run there, we `or` in a mask provided by the runtime (as an absolute symbol) telling us whether the unwinders can cope. When deploying only for newer OSs, we can still hard-code the bit-set for greater efficiency. rdar://79357449
Introduce a new command-line flag `-swift-async-fp={auto|always|never}`
that controls how code generation sets the Swift extended async frame
info bit. There are three possibilities:
* `auto`: the default, which determines how to set the bit based on
deployment target, either statically or dynamically via
`swift_async_extendedFramePointerFlags`.
* `always`: always set the bit statically, regardless of deployment
target.
* `never`: never set the bit, regardless of deployment target.
Patch by Doug Gregor <[email protected]>
Differential Revision: https://reviews.llvm.org/D109392
This matches what we did before the introduction of dynamic swift_async_extendedFramePointerFlags usage
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