[PATCH] D79053: [SVE] Fix invalid use of VectorType::getNumElements() in Value Tracking

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

It's not obvious what DemandedElts should actually mean for a scalable vector.  How do you decide whether elements past the minimum length are actually demanded?  I can think of the following possibilities:

1. The elements beyond the minimum are always demanded
2. The demanded elements is a repeating pattern
3. We shouldn't use DemandedElts for scalable vectors at all.

It probably makes sense to just choose the last one until we actually have some real-world testscases that would benefit.  Also saves some implementation work short-term.

Given that, it probably makes sense to only construct DemandedElts for FixedVectorTypes, and just use `APInt(1, 1)` for scalable vectors.



================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:1946
+  if (isa<ConstantVector>(V) && !EC->Scalable) {
+    const auto *CV = cast<ConstantVector>(V);
+    assert((CV->getNumOperands() == EC->Min) &&
----------------
No reason to check Scalable; a ConstantVector is never scalable anyway.


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