[LLVMdev] [RFC] CGContext skeleton implementation

[Dan Gohman]

On Mon, Dec 2, 2013 at 4:25 PM, Andrew Trick <atrick at apple.com> wrote:

>
> On Nov 25, 2013, at 4:40 PM, Dan Gohman <dan433584 at gmail.com> wrote:
>
> > Hello llvm-dev,
> >
> > Following up on the "CodeGen Context" discussion that was started,
> attached is patch which implements a pretty minimal skeleton of a CGContext
> implementation. The goal is to allow the newly added subtarget attributes
> on functions to be made available to codegen so that codegen can actually
> switch subtargets within a Module.
> >
> > Comments and questions are invited herewith.
> >
> > There has been some disagreement as to what to name this class. I have
> stuck with "CGContext" for now, though I'm absolutely not attached to that
> name.
> >
> > It demonstrates where the CGContext state lives, how code can access it
> (in X86ISelLowering.cpp), how it reads the attributes from the IR, and how
> it gets cached.
> >
> > One thing that it doesn't yet do is hook up to the target-specific
> subtarget feature interpretation. For example, it would need to hook up to
> the x86 subtarget code which knows that, for example, sse4.2 implies
> sse4.1, and so on. However, I'm sending this out now so that the other
> parts of the patch can get some feedback.
>
> Hi Dan,
>
> This basic idea makes sense to me. It is consistent with my understanding
> of Bill's proposal. What I don't understand is why CGContext is tied to to
> CodeGen objects and initialized at ISelLowering. We should be able to get a
> CGContext from a TargetMachine by providing a Function.
>

> IR passes like the LoopVectorizer and LSR also require subtarget
> information. This was formalized by TargetTransformInfo. Now any pass using
> TargetTransformInfo needs to respect the current Function attributes, which
> means reinitializing subtarget info. I also think that IR passes should use
> the same interface as SD/MI passes to query codegen modes, rather than
> directly querying Function attributes. For that reason, they should also
> use CGContext. It should be possible to override those modes with command
> line options, but I don't think we should have ad-hoc overrides buried
> within passes.
>

In my current patch, the CGContext instances are kept within
MachineModuleInfo instance. You're right that this is a Codegen-oriented
class. What would you think about having the CGContext instances kept in
the LLVMContext? This would also allow them to be cached in the same way,
and it would make them available to any part of the middle or backend that
wanted them.


>
> I think it would be cleanest if the IR pass pipeline had an explicit point
> at which the subtarget gets initialized--say a SubtargetInfo pass. IR
> passes that run early can still get architecture defaults from
> TargetTransformInfo, but can't make any subtarget specific queries. IR
> passes that run after this point should be able to make CGContext queries,
> either directly or via TargetTransformInfo (I'm not sure there is any real
> advantage in treating TargetTransformInfo as an analysis).
>

Dividing the optimization pipeline into "before" and "after" subtarget
information is a bit beyond the scope of what I'm looking to do here. If
CGContext is moved to LLVMContext (and perhaps renamed), then it can be
available to any pass, and the decision of when to introduce subtarget
information can be made at a higher level.

Dan
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