[llvm] [BOLT][RISCV] Preserve debug metadata when rewriting call pairs (PR #221116)

[Alexander Wilson via llvm-commits]

rdtscp wrote:

> I think what do you want to fix is similar to this #214721 ?

I think that my (work-in-progress) https://github.com/llvm/llvm-project/pull/221245 is related to that issue (caveat that I don't fully understand that yet. Codex claims its necessary to fix BOLT for my use-case).

This PR is just about fixing the metadata on NOP/AUIPC instructions.
As I understand it, `AUIPC + JALR` instruction pairs are converted to `NOP + PseudoCALL` (for BOLT to create the CFG), and then during re-emit of the re-ordered code the `PseudoCALL` gets translated back to `AUIPC + JALR`, but the NOP still exists and shifts the subsequent instructions by 4 bytes.

As such, we need to move the metadata from the original `AUIPC` to the prepended `NOP` so that BOLT knows where/how to emit the correct DWARF boundaries/source location and offset metadata in the re-emitted binary.

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This prepended `NOP` already exists in trunk, but seems inefficient to me. Seems like in order to reorder some blocks we are introducing a 1 instruction NOP cost each time. Not sure if this matters in practise, but changing this seems like more work and I don't fully understand the implications

https://github.com/llvm/llvm-project/pull/221116