[LLVMdev] Status of pre-legalize vector changes

[Dan Gohman]

On Mon, Jun 11, 2007 at 03:39:40PM -0700, Chris Lattner wrote:
> On Mon, 11 Jun 2007, Dan Gohman wrote:
> > Also, CopyToReg and CopyFromReg lowering/legalization rely on being able to
> > create vector-of-vector types. For example, an <8 x double> vector is bitcasted
> > to (effectively) <4 x <2 x double>> for x86, allowing EXTRACT_ELEMENT to pull
> > out whole legal vectors at a time instead of just individual elements, for
> > example.
> 
> Hrm, now that you mention it, I do remember that.  I'd file that into a 
> category of "evil hacks", not something that we really want to support. 
> To me, I think it would be much better to fix this directly, rather than 
> complicate your implementation.
> 
> In particular, you should be able to introduce a new [V]CONCAT_VECTOR 
> node, which takes two input vectors and yields an output vector that has 
> length equal to the sum of the input vectors.  Likewise, instead of the 
> extract_element hack, we should have an EXTRACT_SUBVECTOR node of some 
> sort.
> 
> Since no target support these nodes, they would all be expanded by 
> legalize.
> 
> Note that this change is logically independent of the rest of your change, 
> so you could do this on mainline, as a first step to getting the bigger 
> change in.

Ok, these are now created. The new opcodes aren't very general yet; currently
they're only suitable for pre-legalize use; they're only currently needed for
lowering CopyToReg and CopyFromReg. Once the rest of the pre-legalize vector
changes go in, these opcodes will loose the leading 'V' in their names, and
then it'll be easier to extend them to other purposes.

> > While my original patch made the extended ValueType table explicitly
> > hold vector element types and vector lengths, I had converted it to hold
> > Type*, and ran into problems because VectorType doesn't permit the element
> > types to be vectors. It seems unfortunate to give up using Type* just because
> > of these few cases, but at the moment I don't have any simpler alternatives.
> 
> Vectors of vectors really is a hack.  Type*'s seems like a good way to go.

Agreed.

> > Putting the table for extended ValueTypes in SelectionDAG seems quite involved.
> > There are a lot of places that use the MVT-namespace functions, and making
> > sure they all have access to a SelectionDAG object in order to use them is a
> > lot of changes. It's tempting to make the table be a ManagedStatic, in
> > VMCore/ValueTypes.cpp, not unlike the ManagedStatic objects in VMCore/Type.cpp,
> > for example.
> 
> The problem with this approach is that it makes it much harder to codegen 
> two different functions in parallel on different threads.  This isn't 
> something we do today, but is something we want to do eventually. 
> Requiring synchronization in such a critical datastructure would be 
> badness...

The badness could be mitigated somewhat with readers-writer locks, because
the common case for this table would be that it would get a few entries added
to it right away and then see very few updates, if any, for the rest of its
lifetime.

I'll take another look at eliminating the static table though.

> That said, I don't know just how horrible the impact is on the code. 
> Maybe there is another way?  Maybe the value table should be a first class 
> datastructure that SelectionDAG contains.  This would let you do things 
> like:
> 
> SD.getValueTable().getNumVectorElements(VT)
> 
> but it would also allow clients to grab a reference to the valuetable and 
> call into it directly if more convenient.  Would this help at all?

Not really. The problem I saw was clients that don't have the SelectionDAG
passed into them. Passing around handles to this new kind of object everywhere
wouldn't be any easier than passing around handles to the DAG itself.

> One nice thing about this is that the ValueTable datastructure would be 
> very simple, and moving all the methods to operate on a value table would 
> be straight-forward.  If ValueTable was a simple class, then tblgen could 
> instantiate it (f.e.) and not populate it with any of the funny MVT's.

You mean like MVT::iAny? Those are still just simple enum values; they don't
live in the table.

Dan