[llvm-commits] [PATCH] PR889: devirtualize Value::printCustom

[Jay Foad]

On 14 November 2011 09:53, Chandler Carruth <chandlerc at google.com> wrote:
> I don't think this is an acceptable way to approach this. You're still
> adding a dependency on CodeGen to VMCore. You've just wrapped it up into a
> header file dependency instead of a link-time dependency. I think this is
> likely to make our already troublesome layering story significantly worse.
> VMCore should not know of or need anything CodeGen.

VMCore defines a class hierarchy rooted at Value. CodeGen wants to
extend that hierarchy with a new subclass, PseudoSourceValue. All
these classes of value want to be able print themselves (for use in
debugging dumps if nothing else). If this is to be done without VMCore
knowing anything about CodeGen then a virtual method seems like a very
good way of doing it.

The point of the PR is that we want to do away with virtual methods to
save space, because we won't need a vtable pointer in each object.
Instead we can do manual dispatch on the object's ValueID. But I can't
think of a good way for independent libraries (like CodeGen) to hook
into this dispatch mechanism. Maybe they could register themselves
with VMCore, saying "I want to extend the Value hierarchy with a new
class with this ID"; but that doesn't sound like it would be very
clean or efficient.

So unless anyone has any bright ideas, it seems to me that this PR is
fundamentally at odds with LLVM's "already troublesome layering
story".

Jay.