[PATCH] D63429: [DebugInfo] Avoid adding too much indirection to pointer-valued variables

[Jeremy Morse via Phabricator via llvm-commits]

jmorse created this revision.
jmorse added reviewers: aprantl, probinson, vsk.
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This patch addresses PR41675 [0], where a stack-pointer variable is dereferenced too many times by its location expression, presenting a value on the stack as the pointer to the stack.

The difference between a stack *pointer* DBG_VALUE and one that refers to a value on the stack, is currently the indirect flag. In the below, the former is the address of, the latter the value in %stack_loc:

  DBG_VALUE %stack_loc, $noreg, !1, !DIExpression()
  DBG_VALUE %stack_loc, 0, !1, !DIExpression()

When the prologepilog pass runs, the stack references are replaced with complex expression opcodes:

  DBG_VALUE $rsp, $noreg, !1, !DIExpression(DW_OP_plus_uconst, 4)
  DBG_VALUE $rsp, 0, !1, !DIExpression(DW_OP_plus_uconst, 4)

Unfortunately, the DWARF backend interprets both of these as being memory locations. For the second DBG_VALUE this is because it's explicitly indirect, but for the first it's because there's a complex expression without DW_OP_stack_value.

Get around this by detecting empty-to-non-empty DIExpression transitions in prologepilog, and adding DW_OP_stack_value to them. This ensures that pointers-to-stack-locs are interpreted as implicit locations, which is what they are.

For the edge case where the offset is zero and the location could be a register location, DIExpression::prepend will still generate opcodes, and thus DW_OP_stack_value must still be added.

[0] https://bugs.llvm.org/show_bug.cgi?id=41675


Repository:
  rL LLVM

https://reviews.llvm.org/D63429

Files:
  lib/CodeGen/PrologEpilogInserter.cpp
  test/DebugInfo/MIR/X86/prolog-epilog-indirection.mir

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