[llvm-dev] Limited use types in the back end

[Eli Friedman via llvm-dev]

I’m not sure I understand the difficulty here.  Normally, if you have an instruction which has multiple operand/result registers, you just make the SelectionDAG node have multiple operand/result values.  If there are weird register allocation constraints, you can handle that in ISelDAGToDAG.  (There are a few ARM instructions that expect multiple registers in ascending order, like vtbl and vld4/vst4.)

If you need inline asm operands/results with an illegal type, that’s sort of an independent issue.  x86 uses a fake register class to handle the “A” constraint, which refers to the register pair RAX/RDX. (See X86TargetLowering::getRegForInlineAsmConstraint).  If that doesn’t work in your case, not sure what I’d do off the top of my head; maybe the code for lowering inline asm could be extended.

-Eli

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Nemanja Ivanovic via llvm-dev
Sent: Monday, January 27, 2020 8:59 AM
To: llvm-dev <llvm-dev at lists.llvm.org>
Subject: [EXT] [llvm-dev] Limited use types in the back end

I am hoping that someone can offer advice on a somewhat unusual issue that I am facing with the SDAG. Namely, I am trying to implement some custom operations that do very specific things on multiple registers at a time. The operations themselves will simply be intrinsics since there are no equivalent operations in IR/SDAG. However, handling the types seems rather tricky.

One approach I tried is to create a register class that has the wide registers with proper sub registers and then telling the SDAG that the correspondingly wide type can go into those registers. While this works, it has a very unfortunate side effect that the type legalizer leaves any node with such a type untouched and I have to mark all operations as non-legal (mostly Expand).
For example, I could say that the type v8i64 can go into these registers and then I can use the type for my intrinsics. However, the type legalizer will leave all nodes with this result/operand type alone which is not at all what I want.

Then I tried the opposite approach - just custom lower only specific nodes that have this result type and let the type legalizer handle all the others normally. This works quite well except if I want to expose those custom instructions through inline asm. The DAG builder complains if I am trying to assign one of these wide registers to a value with the wide type because it assumes that the wide value will need to be broken up.

I suppose I could define a new type for the IR/SDAG and use it, but that seems like a super pervasive approach.

So either direction I go in seems to have a major drawback.
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