tree 587561b3c1f5e984ee76d8d3a432a88120e346f7
parent 65507c48ddcdf3e71c0bb0d1c85791c955e1804f
author Adrian Prantl <aprantl@apple.com> 1613084650 -0800
committer Copybara-Service <copybara-worker@google.com> 1613726025 -0800

Support dereferencing a DWARF scalar stack value

Swift async functions receive function arguments inside a
heap-allocated data structure, similar to how ObjC block captures or
C++ coroutine arguments are implement. In DWARF they are described
relative to an entry value that produces a pointer into that heap
object. At typical location looks like

DW_OP_entry_value [ DW_OP_reg14 ] DW_OP_deref DW_OP_plus_uconst 32 DW_OP_deref

This allows the unwinder (which has special ABI knowledge to restore
the contents of r14) to push the base address onto the stack thus
allowing the deref/offset operations to continue. The result of the
entry value is a scalar, because DW_OP_reg14 is a register location —
as it should be since we want to restore the pointer value contained
in r14 at the beginning of the function and not the historical memory
contents it was pointing to. The entry value should restore the
address, which is still valid, not the contents at function entry.

To make this work, we need to allow LLDB to dereference Scalar stack
results like load addresses, which is what this patch
does. Unfortunately it is difficult to test this in isolation, since
the DWARFExpression unit test doesn't have a process.

Differential Revision: https://reviews.llvm.org/D96549

GitOrigin-RevId: 188b0747c1664d962e94f00b5e3caac529fa1e26