[PATCH] D36795: [SystemZ] Increase number of LOCRs emitted by passing regalloc hints

[Jonas Paulsson via Phabricator via llvm-commits]

jonpa added a comment.

Ping!

This is my original post on llvm-dev:

> Hi,
> 
> I am curious if it would be possible to use reg-alloc hints to improve code generation for SystemZ. The background is that I ran into a regression which seems to relate to code generation for conditional register moves.
> 
> The SystemZ backend uses a GRX32 register class for a LOCRMux pseudo instroction (Load On Condition of Register), in order to utilize all 32bit registers optimally. However, depending on the register assignment this pseudo will become a single load-on-condition instruction only if both source and dest registers are either low or high parts. Otherwise, LOCRMux will expand to a compare/jump sequence, which is of course less desirable. LOCR can only handle two low-parts, and LOCFHR can only handle two high-parts (GRX32 is the union of these two reg classes).
> 
> In order to increase the number of LOCR/LOCFHRs generated, I would like to tell regalloc something like  "If src reg of an LOCRMux is high, try to make dst reg high", and similarly for the case where one register is in the low part.
> 
> I am not sure if it is possible to do anything about this in a simple manner, and would appreciate any help.
> 
> Thanks,
> 
> Jonas

I have now experimented a while with this, and found some different ways of handling this, which I need some feedback on. In particular, it seems promising to use "hard hints", by which I mean changing the AllocationOrder so that only hinted registers are returned. This seems to work, but it is a good idea?

If this is not acceptable, another option might be to use the updateRegAllocHints() hook by *constraining the regclass* of the virtreg directly. This is not the documented purpose of this hook, wo I wonder if it would make sense generally?

The goal is to get rid of the RISB (rotate and insert emitted by copyPhysReg(), and the results can be found in the table above, indicating the effectiveness of the various methods.


https://reviews.llvm.org/D36795