[s390x] Fix SP unwind rule for the tail-call ABI #9725
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On s390x, the unwound SP is always at current CFA - 160. Therefore, the default rule used on most other platforms (which sets the unwound SP to the current CFA) is incorrect, so we need to provide an explicit DWARF CFI rule to unwind SP. With the platform ABI, the caller's SP is always stored in the register save area like other call-saved GPRs, so we can simply use a normal DW_CFA_offset rule. However, with the new tail-call ABI, the value saved in that slot is incorrect - it is not corrected for the incoming tail-call stack arguments that will have been removed as the tail call returns. To fix this without introducing unnecessary run-time overhead, we can simply use a DW_CFA_val_offset rule that will set the unwound SP to CFA - 160, which is always correct. However, the current UnwindInst abstraction does not allow any way to generate this DWARF CFI instruction. Therefore, we introduce a new UnwindInst::RegStackOffset rule for this purpose. Fixes: bytecodealliance#9719
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On s390x, the unwound SP is always at current CFA - 160. Therefore, the default rule used on most other platforms (which sets the unwound SP to the current CFA) is incorrect, so we need to provide an explicit DWARF CFI rule to unwind SP.
With the platform ABI, the caller's SP is always stored in the register save area like other call-saved GPRs, so we can simply use a normal DW_CFA_offset rule. However, with the new tail-call ABI, the value saved in that slot is incorrect - it is not corrected for the incoming tail-call stack arguments that will have been removed as the tail call returns.
To fix this without introducing unnecessary run-time overhead, we can simply use a DW_CFA_val_offset rule that will set the unwound SP to CFA - 160, which is always correct. However, the current UnwindInst abstraction does not allow any way to generate this DWARF CFI instruction. Therefore, we introduce a new UnwindInst::RegStackOffset rule for this purpose.
Fixes: #9719