[LLVMdev] [RFC] CGContext skeleton implementation

[Chandler Carruth]

On Thu, Dec 5, 2013 at 7:36 PM, Andrew Trick <atrick at apple.com> wrote:

> It might nice if scalaropts did not need to link against libTarget—we
> could easily avoid introducing more dependence on libTarget if that’s a
> goal.
>

We already have that state today in the overwhelming majority of the cases.
The pass manager built by the PassManagerBuilder doesn't need a target at
all. Yes, there is CodeGenPrepare which violates all manner of rules, but
thus far it has been viewed as a hack, and something that should go away
because, at a very fundamental level, it *is* a lowering pass.


> Either way, we need an IR-level API that requires subtarget initialization.
>

Why? The only concrete use case I can come up with is lowering, but you
claim that you aren't trying to do lowering in the IR -- If you can't make
reasonable decisions without a concrete subtarget, then I claim you *are*
lowering. This is why I suggested that this would be a very significant
change in design from the direction that LLVM is currently moving.


I can see the need for target *heuristics* in an optimization pass (not
necessarily canonicalization) that is not doing lowering. But by definition
of being heuristics, there is an obvious way to implement and test them in
a target independent manner. When the target information is no longer a
heuristic and cannot be stubbed out in a target independent manner, I claim
that the transformation it is guiding has ceased to be target independent
as well and doesn't belong in the IR.


But all of this should absolutely be discussed in a separate thread (or in
person) as it is largely orthogonal to allowing the TargetMachine to react
appropriately to function attributes.
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