llvm-project

e897cb13 - [lldb] Provide lr value in faulting frame on arm64 (#138805)

[lldb] Provide lr value in faulting frame on arm64 (#138805) When a frameless function faults or is interrupted asynchronously, the UnwindPlan MAY have no register location rule for the return address register (lr on arm64); the value is simply live in the lr register when it was interrupted, and the frame below this on the stack -- e.g. sigtramp on a Unix system -- has the full register context, including that register. RegisterContextUnwind::SavedLocationForRegister, when asked to find the caller's pc value, will first see if there is a pc register location. If there isn't, on a Return Address Register architecture like arm/mips/riscv, we rewrite the register request from "pc" to "RA register", and search for a location. On frame 0 (the live frame) and an interrupted frame, the UnwindPlan may have no register location rule for the RA Reg, that is valid. A frameless function that never calls another may simply keep the return address in the live register the whole way. Our instruction emulation unwind plans explicitly add a rule (see Pavel's May 2024 change https://github.com/llvm/llvm-project/pull/91321 ), but an UnwindPlan sourced from debug_frame may not. I've got a case where this exactly happens - clang debug_frame for arm64 where there is no register location for the lr in a frameless function. There is a fault in the middle of this frameless function and we only get the lr value from the fault handler below this frame if lr has a register location of `IsSame`, in line with Pavel's 2024 change. Similar to how we see a request of the RA Reg from frame 0 after failing to find an unwind location for the pc register, the same style of special casing is needed when this is a function that was interrupted. Without this change, we can find the pc of the frame that was executing when it was interrupted, but we need $lr to find its caller, and we don't descend down to the trap handler to get that value, truncating the stack. rdar://145614545