[PATCH] D65368: [DebugInfo] LiveDebugValues: Don't drop fragment information when restoring spills

[Adrian Prantl via Phabricator via llvm-commits]

aprantl added inline comments.


================
Comment at: llvm/lib/CodeGen/LiveDebugValues.cpp:860
+  // FIXME: Don't create a spill transfer if there is a complex expression,
+  // because we currently cannot recover the original expression on restore.
   for (unsigned ID : OpenRanges.getVarLocs()) {
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jmorse wrote:
> aprantl wrote:
> > Ah.. that's why?
> > 
> > Shouldn't we then just copy the original expression? That would be less code and more obvious.
> IIRC, when restored, the only DBG_VALUE available to the restore-code is the spilt-location expression rather than the original. We'd have to track additional information about the original expression in the LiveDebugValues data structures -- which is fine, but IMHO too much new material to pull into LLVM-9.
> 
> We could also strip the additional-offset from the expression, but I worry about that too, see main comment from 2019-08-04 above.
> 
> (Looks like I missed saying "crash mitigation" in the review description, curses, I'll edit that in)
I understand the constraints now. This is a new expression that just describes the spill. Due to the concatenative nature of DWARF expressions, I believe that you still should be able to just append the operations describing the spill at the end of the original expression (but before DW_OP_llvm_fragment/DW_OP_stack_value) and be safe. That's how we do it in salvageDebugInfo for example. Is this situation any different?


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D65368/new/

https://reviews.llvm.org/D65368