[all-commits] [llvm/llvm-project] cb714e: [DebugInfo][InstrRef] Avoid producing broken DW_OP...

[Jeremy Morse via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: cb714e74cc0efd5bfdb3e5e80978239425bd83d4
      https://github.com/llvm/llvm-project/commit/cb714e74cc0efd5bfdb3e5e80978239425bd83d4
  Author: Jeremy Morse <jeremy.morse at sony.com>
  Date:   2025-01-23 (Thu, 23 Jan 2025)

  Changed paths:
    M llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
    A llvm/test/DebugInfo/MIR/InstrRef/deref-spills-with-size-too-big.mir

  Log Message:
  -----------
  [DebugInfo][InstrRef] Avoid producing broken DW_OP_deref_sizes (#123967)

We use variable locations such as DBG_VALUE $xmm0 as shorthand to refer
to "the low lane of $xmm0", and this is reflected in how DWARF is
interpreted too. However InstrRefBasedLDV tries to be smart and
interprets such a DBG_VALUE as a 128-bit reference. We then issue a
DW_OP_deref_size of 128 bits to the stack, which isn't permitted by
DWARF (it's larger than a pointer).

Solve this for now by not using DW_OP_deref_size if it would be illegal.
Instead we'll use DW_OP_deref, and the consumer will load the variable
type from the stack, which should be correct.

There's still a risk of imprecision when LLVM decides to use smaller or
larger value types than the source-variable type, which manifests as
too-little or too-much memory being read from the stack. However we
can't solve that without putting more type information in debug-info.

fixes #64093



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