[llvm-dev] Handling argument for an intrinsic

[Louis Li via llvm-dev]

Hi Philip, thanks for the response. No, I don't need to vary the pseudo op
-- I did follow the other intrinsics but what happens now (after tinkering
some more) is that my instruction actually gets optimized out (after
debugging by showing the selection dag after various rounds of
optimization). It might help if I send a patch along -- I uploaded it to
phabricator with a link to selectiondagbuilder.

https://reviews.llvm.org/D27503#58aa05e8

On Fri, Dec 2, 2016 at 5:35 PM Philip Reames <listmail at philipreames.com>
wrote:

> On 12/01/2016 04:01 PM, Louis Li via llvm-dev wrote:
>
> Hi, I'm trying to implement a target-agnostic intrinsic, first targeting
> X86. I'm trying to map the intrinsic SD node to an instruction with a
> certain target opcode that I've introduced. The issue that I'm running into
> is what the correct way to lower the argument is. I've done a couple loops
> on the docs so any help would be appreciated!
>
> I'm really confused by the way you're asking the question.  Assuming
> you're adding code to SelectionDAGBuilder, handling the argument should
> just be a getValue(Value*) call.  Take a look at the lowering for ctlz (or
> one of many other examples), how are your arguments different than what's
> done here?  Do you need to vary the psuedo op emitted depending on the
> argument or something like that?
>
>
> Some options I've explored but have been missing some crucial step/concept:
> - tablegen matching?
> - SelectionDAGBuilder.cpp + BuildMI call (but then this runs into the
> issue of lowering the argument correctly - not sure if that's feasible at
> this point in instruction selection)
> - X86ISelLowering -- same confusion as above
>
> Here's how the dag currently looks (before erroring out, since I haven't
> handled the intrinsic)
>
> https://drive.google.com/file/d/0B5G4m_mQneSjRWQzRVltdDJOQnM/view?usp=sharing
> Would it be problematic if I somehow extracted the pointer address arg to
> the intrisic from TargetConstant and passed it as MachineOperand to the
> MachineInstruction?
>
> Opcode definition (Target/Target.td)
> ==============
> +def PATCHABLE_LOG_CALL : Instruction {
> +  let OutOperandList = (outs);
> +  let InOperandList = (ins unknown:$entry);
> +  let AsmString = "# XRay Custom Log.";
> +  let usesCustomInserter = 1;
> +  let hasSideEffects = 1;
> +}
>
> Example IR for the graph above
> ===========
> define i32 @caller() nounwind noinline uwtable
> "function-instrument"="xray-always" {
>     %logentryptr = alloca i8
>     call void @llvm.xray.customlog(i8* %logentryptr)
>     ret i32 0
> }
>
> declare void @llvm.xray.customlog(i8*)
>
>
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