llvm dev - Dec 2016 - [Repost] Handling arguments for an intrinsic

(reposting because my coworker Dean Berris said sending the thread from my
corporate email would put it in the spam folder for a lot of people)
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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 issues that I'm running
into
are 1. what the correct way to lower the argument is 2. not getting my
opcode optimized out. I've done a couple loops
on the docs so any help would be appreciated!

Here's a patch on phabricator:
https://reviews.llvm.org/D27503

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
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