[lld] [llvm] [ICF] Add a NOP after branch in ICF thunk to improve debugability (PR #154986)

[David Blaikie via llvm-commits]

dwblaikie wrote:

> Thanks for pointing that out!
> 
> But if you have three consecutive one-instruction functions (which would be a common case if you enable ICF safe thunks), they will be concat into one sequence: https://godbolt.org/z/GhrYP8njv
> 
> ```
> 0x0000000000000000      1     12      1   0             0       0  is_stmt prologue_end
> 0x0000000000000010      2     12      1   0             0       0  is_stmt prologue_end
> 0x0000000000000020      3     12      1   0             0       0  is_stmt prologue_end
> 0x0000000000000021      3     12      1   0             0       0  is_stmt end_sequence
> ```
> 
> The problem we have here is, for ICF thunks to be probably symbolicated, each function needs to be its own sequence when we enable `-mllvm -emit-func-debug-line-table-offsets`: https://godbolt.org/z/qbndoGWbM
> 
> ```
> 0x0000000000000000      1     12      1   0             0       0  is_stmt prologue_end
> 0x0000000000000000      1     12      1   0             0       0  is_stmt end_sequence
> 0x0000000000000010      2     12      1   0             0       0  is_stmt prologue_end
> 0x0000000000000010      2     12      1   0             0       0  is_stmt end_sequence
> 0x0000000000000020      3     12      1   0             0       0  is_stmt prologue_end
> 0x0000000000000021      3     12      1   0             0       0  is_stmt end_sequence
> ```
> 
> In this case, we can just advance PC, as the PC will go into other thunks.

I'm pretty sure this table is incorrect/buggy - it contains duplicate entries for the same offset.

It describes zero-length, not one-length sequences.

3 single-instruction functions with separate sequences should be emitted like this:
```
0x0000000000000000      1     12      0   0             0       0  is_stmt prologue_end
0x0000000000000001      1     12      0   0             0       0  is_stmt end_sequence
0x0000000000000001      2     12      0   0             0       0  is_stmt prologue_end
0x0000000000000002      2     12      0   0             0       0  is_stmt end_sequence
0x0000000000000002      3     13      0   0             0       0  is_stmt prologue_end
0x0000000000000003      3     13      0   0             0       0  is_stmt end_sequence
```

https://github.com/llvm/llvm-project/pull/154986