[cherry-pick][lldb][swift] Use frame formation as a guide for async unwinding #10687
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This patch changes how the swift language runtime obtains the async context of the currently executing function. The old strategy consists of querying unwind plans for what they believe the location of the async register is. This proved problematic in cases where the register is not saved directly, e.g. in arm64e, where the register is signed and moved to a scratch register prior to being saved. The new strategy relies on ABI guarantees: the async context is _always_ saved at the stack slot immediately beneath where FP is saved. As long as a frame has been formed, we can always access the async context by reading memory @ [CFA-24] (FP is saved in [CFA-16]). Before the frame is formed, the async register can be accessed directly. In other words, the main problem this patch has to solve is detecting the point at which a frame has been fully formed. This is accomplished by scanning the first few Rows of the assembly unwind plan, until a Row is found where FP's location is "AtCFAPlusOffset". The Row immediately after that is used. This is reliable because async prologues follow this pattern in all tested architectures (x86-64, arm64, arm64e): // 0. adjust sp // 1. save lr @ CFA-8 // 2. save fp @ CFA-16 // 3. save async_reg @ CFA-24 << "row immediately after" For frameless functions, we can always use the async register directly. While this patch adds no tests, all of the existing stepping tests and tests inspecting variables exercise the new behavior. (cherry picked from commit d1ac271)
felipepiovezan
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This patch changes how the swift language runtime obtains the async context of the currently executing function.
The old strategy consists of querying unwind plans for what they believe the location of the async register is. This proved problematic in cases where the register is not saved directly, e.g. in arm64e, where the register is signed and moved to a scratch register prior to being saved.
The new strategy relies on ABI guarantees: the async context is always saved at the stack slot immediately beneath where FP is saved. As long as a frame has been formed, we can always access the async context by reading memory @ [CFA-24] (FP is saved in [CFA-16]). Before the frame is formed, the async register can be accessed directly.
In other words, the main problem this patch has to solve is detecting the point at which a frame has been fully formed. This is accomplished by scanning the first few Rows of the assembly unwind plan, until a Row is found where FP's location is "AtCFAPlusOffset". The Row immediately after that is used. This is reliable because async prologues follow this pattern in all tested architectures (x86-64, arm64, arm64e):
// 0. adjust sp
// 1. save lr @ CFA-8
// 2. save fp @ CFA-16
// 3. save async_reg @ CFA-24 << "row immediately after"
For frameless functions, we can always use the async register directly.
While this patch adds no tests, all of the existing stepping tests and tests inspecting variables exercise the new behavior.
(cherry picked from commit d1ac271)