[llvm-dev] How to add a barrier pseudo instruction?

[Matthias Braun via llvm-dev]

> On Sep 19, 2018, at 3:39 AM, Tim Northover via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi Son,
> 
> On Tue, 18 Sep 2018 at 21:01, Son Tuan VU <sontuan.vu119 at gmail.com> wrote:
>> So IIUC, optimization passes in opt do not reorder IR instructions, only passes in llc that move MIR instructions around. Is it correct?
> 
> I don't think active scheduling (i.e. trying to optimize order for
> speed) goes on in IR, but incidental movements will definitely happen.
> 
>> On the back-end (llc) side, hasSideEffects might prevent some reordering. But I just learn about TargetInstrInfo::isSchedulingBoundary. Can you tell me what are the differences between the two please?
> 
> Not in detail, I'm afraid. I'd never heard of isSchedulingBoundary
> before. It looks like it might improve matters if you really want to
> enforce no movement, but it wasn't designed for the purpose so I
> couldn't guarantee it.

I believe isSchedulingBoundary is a tool to designate scheduling areas for the machine scheduler. I don't think it will prevent movement in all cases. I would not expect things like MachineLICM or SelectionDAG combines to look at it when they deal with instructions without side effects or memory operands...

> 
> Cheers.
> 
> Tim.
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