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

[Nemanja Ivanovic via llvm-dev]

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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