[llvm-dev] [GlobalISel] Extended inline assembler support

[Daniel Sanders via llvm-dev]

Hi Konstantin,

Thanks for working on this!

> On 6 Apr 2020, at 03:11, Konstantin Schwarz via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi!
> 
> So far, GlobalISel only supports very basic inline assembler constructs (no input/output operands, only simple memory clobbers).
> 
> In [0], I'm adding support for generic register, immediate, memory and clobber constraints.
> The code is more or less a direct port from the handling in SelectionDAGBuilder.
> 
> Before moving on with target specific constraints, I'd like to discuss the general strategy for target hooks in GlobalISel.
> While most of these hooks on the SelectionDAG side live in the TargetLowering class, there is no corresponding GlobalISel subclass of the TargetLoweringBase class yet.
> Instead, the (one) existing GlobalISel hook currently lives in TargetLoweringBase.
> 
> For lowering the target specific constraints, we'll have to add a new API that creates (G)MIR during IR translation.
> Personally I would prefer to have an InlineAsmLowering class, similar to the CallLowering class, which implements the generic algorithm and from which targets can inherit.
> Most of the implementation in [0] could then be moved to that class.

+1. The SelectionDAG implementation is scattered over a few places and each time I've gone to add something I've missed a bit somewhere. Having it all in a lowering class for inline assembly would be great. It also means there's a clear starting point for targets that are adding support for it.

> However, this would add yet another class that needs to be wired up in the subtarget class. Is this something we would like to avoid?

I don't see a problem with that. If we allow nullptr then it becomes a nice optional component that targets can add when they need to.

FWIW, there might be an argument for putting it in the LLVMTargetMachine (fewer instances) and having the users provide the subtarget when they use it but I'd lean towards putting it in the subtarget for consistency with the rest of GlobalISel.

> Is there a better place where target specific GlobalISel hooks should be added?
> 
> Thanks for your feedback!
> 
> Best regards,
> 
> Konstantin
> 
> [0] https://reviews.llvm.org/D77535
> 
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