[LLVMdev] How to 'define and use' a LOOP intrinsic that takes "iteration count" and the "label" to jump to ?

[RAVI KORSA]

Thank you for pointing to the PPCCTRLoops. That's a good idea. Yes I
understand that OPT passes will have problems with my intrinsic, since they
do not know that  it's a terminator instruction. However in the backend, I
have defined the instruction with "isBranch=1, isTerminator=1". This can
prevent the backend from reordering the instruction. Just out of curiosity,
why can't we have an intrinsicProperty that says "this intrinsic is a
terminator or a branch" and allow it to take a label ?. Such an
intrinsicProperty will allow all the OPT passes to understand any new
branch or loop kind of intrinsics.

Thanks
Ravi.


On Thu, Feb 27, 2014 at 2:13 AM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> > From: "RAVI KORSA" <ravi.korsa at gmail.com>
> > To: llvmdev at cs.uiuc.edu
> > Sent: Wednesday, February 26, 2014 2:14:25 PM
> > Subject: [LLVMdev] How to 'define and use' a LOOP intrinsic that takes
> "iteration count" and the "label" to jump to ?
> >
> >
> >
> >
> > I have defined the intrinsic as
> > def int_loop: Intrinsic<[],[llvm_i8_ty, llvm_empty_ty],[]>;
> >
> >
> > and also got the Codegen backend support in Instructioninfo.td file.
> >
> >
> > Then created a .ll file to test it.
> >
> > The .ll file is like this
> >
> > declare void @llvm.loop(i8, label)
> >
> > define void @fn() nounwind readnone {
> > entry:
> > .....
> > .....
> > call void @llvm.loop(i8 10, label %entry)
> > ret void
> > }
>
> I suspect that you'll run into several problems doing this. For one thing,
> LLVM has builtin assumptions about what kinds of instructions act as block
> terminators, and your intrinsic call is not one of them. I recommend
> looking at how the PowerPC counter-based loops are implemented. PowerPC
> uses a special intrinsic (inserted by the target-specific late IR-level
> pass lib/Target/PowerPC/PPCCTRLoops.cpp):
>   def int_ppc_is_decremented_ctr_nonzero : Intrinsic<[llvm_i1_ty], [], []>;
> along with:
>   def int_ppc_mtctr : Intrinsic<[], [llvm_anyint_ty], []>;
>
> and the first intrinsic is used with the regular loop branching
> instructions at the IR level. In the backend, loop branching instructions
> that are fed by that intrinsic are transformed into target-specific SDAG
> nodes (and, then, to the appropriate instructions).
>
> If you have any further questions, please feel free to ask.
>
>  -Hal
>
> >
> >
> > But when I run it with llc, I get the following error message
> >
> > Intrinsic has incorrect argument type!
> > void (i8, label)* @llvm.loop
> > Broken module found, compilation aborted!
> >
> >
> > The 'Module Verifier' is complaining about the 'label' type. Any
> > suggestions on how I go about doing this ? Would appreciate your
> > help.
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > _______________________________________________
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> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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