[PATCH] D123599: [DebugInfo][InstrRef] Describe value sizes when spilt to stack

[Jeremy Morse via Phabricator via llvm-commits]

jmorse created this revision.
jmorse added reviewers: StephenTozer, Orlando.
Herald added subscribers: hiraditya, qcolombet.
Herald added a project: All.
jmorse requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

With exciting new variable locations from instruction referencing comes exciting new ways for locations to be broken. I've run into a sample where LLVM reduces the size of an enumeration value from 32 bits to a single byte, inserting the relevant DW_OP_convert's. Unfortunately, when the single byte is spilt to the stack we describe it's location correctly, but the fact that it's only one byte is omitted. Consumers faced with a stack location and DW_OP_deref quite naturally load 32 bits from the stack instead of 8 bits, which then corrupts the variable value displayed.

The location can be correctly described with DW_OP_deref_size. This patch seeks out the size of the value and the size of the variable, and uses DW_OP_deref_size if they don't match.

While that's fairly simple, doing this stumbles into the awkward problem of whether locations should be:

- A location expression with an implicit deref,
- A location expression with a DW_OP_stack_value,

and we're forced to commit to one of them at this point, because it affects where the DW_OP_deref{,_size} is placed. There are then several different inputs to location-production that need to be handled. The result is a four-case if/else block, which is unfortunate, but is a gnarly problem that should be designed away at a later date.

Other test changes aside from the one added: one test needs the variable size corrected to avoid spurious changes. Another test with a FIXME is fixed (live-debug-values-restore.mir), the problem was that we have to change the expression produced for a stack slot depending on whether there's already a non-empty DIExpression. For an empty expression, adding "DW_OP_uconst, 8" and becoming a location expression with implicit deref is sufficient. But if there's something else in the expression, we have to explicitly add DW_OP_deref so that the other expression operators can operate on the loaded value. Ugly, but here we are.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D123599

Files:
  llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
  llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.h
  llvm/test/DebugInfo/MIR/InstrRef/deref-spills-with-size.mir
  llvm/test/DebugInfo/MIR/X86/live-debug-values-restore.mir
  llvm/test/DebugInfo/MIR/X86/livedebugvalues_load_in_loop.mir

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D123599.422180.patch
Type: text/x-patch
Size: 19761 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20220412/393698eb/attachment.bin>