[llvm-dev] InlineSpiller - hoists leave virtual registers without live intervals

Quentin Colombet via llvm-dev llvm-dev at lists.llvm.org
Mon Nov 4 12:18:42 PST 2019

Hi Alex,

Thanks for reporting this.
Wei worked on the hoisting optimization.

@Wei, could you work with Alex to see what is the problem.


> On Nov 3, 2019, at 5:20 AM, via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 	/// Optimizations after all the reg selections and spills are done.
> 	void InlineSpiller::postOptimization() { HSpiller.hoistAllSpills();
> }
> Seems a problematic function to me, as hoistAllSpills() uses
> TII.storeRegToStackSlot() to insert new spills.
> The problem is, TII.storeRegToStackSlot is allowed to create new virtual
> registers, which can not be allocated a range as this whole thing is called
> _after_ all reg selection is complete.
> If I'm right in this, I do not see how the in-tree target AMDGPU::SI has not
> been affected, as it creates virtual registers in both load and store stack
> operations in SIInstrInfo.cpp - which is where I confirmed to myself that it
> was okay to do so. When compilation broke,
> http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20130812/184331.htm
> l further suggested that the intention is that you can... but I do not see
> how a hoist can ever pass verification/compile correctly. Am I doing
> something incorrectly here? Or are hoistable spills just that rare on GPU
> code that it's never come up?
> As a side note, compiler option "-disable-spill-hoist" (DisableHoisting) is
> referenced in exactly zero places, so if anyone in a similar situation finds
> this post, maybe don't bother testing with that flag just yet. :)
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