[cfe-dev] CPU features handling

[Roman Divacky]

> > I am not sure how to do it though. Can you comment on what you think is
> > the best way to tackle this? We cant link libclangBasic to libLLVMTarget
> > and it seems strange to copy the *GenSubtarget.inc to libBasic. I cant
> > think of any other solution
> 
> There are two basic approaches. It's been a while since I looked at
> this, so I forget enough details to know which one we should pursue.
> 
> The first approach is to extend the target interface so that client's
> of LLVM can interrogate the target. This should be much easier to add
> now that we have the target registry. One downside is it means we
> would be linking in more code, but the work on the target registry
> means that in theory clients can only link in the "target info" and
> not the entire code generator. Theory != practice, currently. Also,
> once we merge clang-cc into clang we will link in everything anyway so
> this is less of a concern.
> 
> The other approach is that we have a tablegen backend which converts
> the target feature information into separate .inc files which we would
> use in clang. The benefit of this approach is that we can massage the
> data into whatever format we want.

I'll go the latter way. I'll modify tblgen to create subtarget features,
ie. FeatureKV[] and SubTypeKV[] and I'll use that to derive the information
we need.

I am not sure how to write the makefile rules though... I'll see what I'll get
and present it here for further comments