[llvm-dev] Understanding and Cleaning Up Machine Instruction Bundles

[Krzysztof Parzyszek via llvm-dev]

On 10/27/2016 4:33 PM, Matthias Braun via llvm-dev wrote:
>
> In fact I am not sure why you would even wait with the finalization and do it
> in a separate pass rather than doing it immediately after forming the bundle.
> Using the pass today does not even work as the MachineVerifier will reject the
> intermediate unfinalized state (missing internal read markers). I'd suggest to get
> rid of the pass and the idea of delegating finalization to an own pass, any objections?

No objections. Also, the existing finalizeBundle function works on 
Hexagon mostly by luck. Hexagon's packet semantics is "mostly" parallel. 
In general all registers are read before any register updates take 
place. The current function works by scanning the bundle sequentially. 
That doesn't cause problems for us, mostly because there is nothing that 
cares enough about the BUNDLE operands.

Actually, in our local branch we add immediate operands to the BUNDLE 
instruction to mark certain properties of the bundle as a whole that are 
of interest to us.  While we can't upstream that code at the moment, we 
would really like to retain that form of use.


> However given that delaying finalization to a pass appears broken/unused it
> seems we could just as well use MachineInstr::iterator instead and remove
> MIBundleOperands. Any objections?

No.


> == Moving to a scheme without repeating the operands in the bundle header ==
>
> I've heard some comments that the long term plan was to move to a scheme where
> the operands inside the bundle are not repeated in a bundle header and instead
> everyone uses an iterator like MIBundleOperands. I could not find any mails
> documenting this, so it would be nice if some people could chime in here if
> that was indeed the plan.

That sounds wrong. To reiterate, a Hexagon's packet is not equivalent to 
a sequence of individual instructions. Packetization on Hexagon can be 
considered irreversible. This may not be the case on other 
architectures.  Iterating over operands within a bundle without 
additional information seems random at best.


-Krzysztof