[llvm-commits] PATCH: new Value::getThruSplat API

Dan Gohman gohman at apple.com
Wed Oct 27 17:42:04 PDT 2010

On Oct 27, 2010, at 4:53 PM, Nick Lewycky wrote:

> On 27 October 2010 10:57, Dan Gohman <gohman at apple.com> wrote:
> On Oct 26, 2010, at 11:34 PM, Nick Lewycky wrote:
> > This patch adds a new method on Value called getThruSplat() which returns the splatted value iff this is a ConstantValue with a splat, or 'this' otherwise. I demonstrate its usage by updating InstructionSimplify to uniformly handle splat vectors alongside ConstantInt values.
> Looks reasonable to me.
> How about "getScalarValue" instead of "getThruSplat"?
> I don't have strong opinions about it though.
> ConstantArray and ConstantStruct aren't scalar and this method could return those. For that matter, it could even return a vector, constant or not.
> My concern is to make very sure that this is the API we really want. Are we sure that "dyn_cast<ConstantInt>(V->getThruSplat())" doesn't do any more work than the minimum (or at least, that the compiler can't optimize away)? And is it okay to put this work on the lookup side? We will do this test more often this way than if we did the work at the point of creation, which is the pattern we follow in the rest of LLVM.

With the current class hierarchy, that dyn_cast really is needed. The splat value
could theoretically be some crazy unfoldable ConstantExpr. It might be nice to
push it into the API though, and have
  ConstantInt *getAsConstantInt();
  ConstantFP *getAsConstantFP();
instead though, if you're going to be doing this a lot.

> The most radical alternative API would be to factor out a common base that represents a ConstantInt+ConstantVector(with splat) and another for ConstantFP+ConstantVector(with splat), then teach ConstantVector to return such a splatted CV from its getter. Sadly if you follow this through to its conclusion, you end up using multiple inheritance in the Constant hierarchy which causes all sorts of forms of pain.

I agree, don't do MI. But I think if you wanted to take this approach, you could
make ConstantInt and ConstantFP capable of having vector types, but not be
subclasses of ConstantVector. ConstantVector would just be for cases where it's
really necessary to have an array of operands. This sounds like a big and scary
change, though there are some reasons it might not have to be.

With this approach, vector ConstantAggregateZero could go away.

Which reminds me; your current patch doesn't handle vector ConstantAggregateZero.


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