llvm dev - Sep 2018 - How to add a barrier pseudo instruction?

Hello all,

I want to add a custom intrinsic to the LLVM IR which would be lowered into
a pseudo instruction since it doesn't correspond to any real instruction
defined by the architecture. The speciality of this intrinsic/pseudo
instruction that it should behave like a scheduling barrier: every
instruction before the intrinsic has to be emitted before the intrinsic,
the same goes for all instructions after the intrinsic, and this should
hold after any optimization in opt and llc.

Can anyone tell me if this is something available in the current LLVM
infrastructure and if so, what should be the correct way to create this
intrinsic/pseudo instruction? Which bit should be set to 1? isBarrier or
hasSideEffects or both? I cannot find the documentation for these 2 bits
other than the comments in Target.td, so I'd like to have some explanation
here.

Thank you so much for your help,

Son Tuan Vu
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Hi,

On Sun, 16 Sep 2018 at 22:02, Son Tuan VU via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> I want to add a custom intrinsic to the LLVM IR which would be lowered into
a pseudo instruction since it doesn't correspond to any real instruction
defined by the architecture. The speciality of this intrinsic/pseudo instruction
that it should behave like a scheduling barrier: every instruction before the
intrinsic has to be emitted before the intrinsic, the same goes for all
instructions after the intrinsic, and this should hold after any optimization in
opt and llc.
That's going to be very difficult. LLVM allows intrinsics to have
unmodelled side-effects that prevent reordering with instructions that
might also have side-effects (including loads and stores). But it
assumes that simple arithmetic instructions (like "add") have no
effect other than what's in the IR so they can be freely moved past
anything.

The difference occasionally comes up when people want begin-benchmark
and end-benchmark intrinsics, but really the whole concept of what's
calculated between two points is pretty fuzzy.
> Which bit should be set to 1? isBarrier or hasSideEffects or both?
hasSideEffects is the closest, with the caveats above.

isBarrier has nothing to do with fences. It's applied to unconditional
branches so that LLVM knows that a basic-block ending with that
instruction won't fall-through.

Cheers.

Tim.

Hi Tim,

Thank you a lot for your reply. So IIUC, optimization passes in opt do not
reorder IR instructions, only passes in llc that move MIR instructions
around. Is it correct?

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?

Thank you very much (again),

Son Tuan Vu


On Mon, Sep 17, 2018 at 12:13 PM Tim Northover <t.p.northover at
gmail.com>
wrote:
> Hi,
>
> On Sun, 16 Sep 2018 at 22:02, Son Tuan VU via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > I want to add a custom intrinsic to the LLVM IR which would be lowered
> into a pseudo instruction since it doesn't correspond to any real
> instruction defined by the architecture. The speciality of this
> intrinsic/pseudo instruction that it should behave like a scheduling
> barrier: every instruction before the intrinsic has to be emitted before
> the intrinsic, the same goes for all instructions after the intrinsic, and
> this should hold after any optimization in opt and llc.
>
> That's going to be very difficult. LLVM allows intrinsics to have
> unmodelled side-effects that prevent reordering with instructions that
> might also have side-effects (including loads and stores). But it
> assumes that simple arithmetic instructions (like "add") have no
> effect other than what's in the IR so they can be freely moved past
> anything.
>
> The difference occasionally comes up when people want begin-benchmark
> and end-benchmark intrinsics, but really the whole concept of what's
> calculated between two points is pretty fuzzy.
>
> > Which bit should be set to 1? isBarrier or hasSideEffects or both?
>
> hasSideEffects is the closest, with the caveats above.
>
> isBarrier has nothing to do with fences. It's applied to unconditional
> branches so that LLVM knows that a basic-block ending with that
> instruction won't fall-through.
>
> Cheers.
>
> Tim.
>
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