[PATCH] D111317: [DebugInfo][InstrRef] Track instructions that write-to-stack after having a spill fused into them

Wed Oct 13 02:47:03 PDT 2021

djtodoro added a comment.

In D111317#3057563 <https://reviews.llvm.org/D111317#3057563>, @jmorse wrote:

> In D111317#3057344 <https://reviews.llvm.org/D111317#3057344>, @djtodoro wrote:
>
>> this actually becomes `DBG_INSTR_REF 2, 0`, right?
>
> Actually no -- when optimisations are occurring with instruction referencing, I'm recording them in the "debugValueSubstitutions" table, which maps "old" instruction numbers onto new. Take a look at the MIR in memory-operand-tracking.mir which is added by this patch; there's a substitution from "2, 0" to "3, 1000000" (the large number signifying the memory operand).
>
> Because instruction referencing gets rid of virtual register references, there's no efficient way to find the debug instructions that use a particular instruction number; perhaps we could make that happen, but I don't think we would gain anything from it. That's why I'm storing records of substitutions in a side-table, they're used to "patch up" the DBG_INSTR_REFs at the end of compilation.
>
> (One downside of this approach is it adds extra steps for humans interpreting the debug-info, because they might have to interpret the substitutions; we could "patch up" DBG_INSTR_REFs as part of the printing process if that becomes a burden).

I see... Thanks for the explanation. Improving the MIR output could be a future-work, but I don't want to block anything here.

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