[PATCH] D113772: [Analysis] Fix getNumberOfParts to return 0 when the answer is unknown

[David Sherwood via Phabricator via llvm-commits]

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When asking how many parts are required for a scalable vector type
there are occasions when it cannot be computed. For example, <vscale x 1 x i3>
is one such vector for AArch64+SVE because no matter how we promote the i3
type we never end up with a legal vector. In BasicTTImpl this means
getNumberOfParts attempts to dereference an invalid cost, which triggers
an assert. This patch changes the BasicTTImpl implementation of
getNumberOfParts to return 0 for such cases, since the definition of
getNumberOfParts in TargetTransformInfo.h states that we can use a
return value of 0 to represent an unknown answer.

Currently, LoopVectorize.cpp is the only place where we need to check for
0 as a return value, because all other instances will not currently
ask for the number of parts for <vscale x 1 x iX> types.

In addition, I have changed the target-independent interface for
getNumberOfParts to return 1 and assume there is a single register
that can fit the type. The loop vectoriser has lots of tests that are
target-independent and they relied upon the 0 value to mean the
answer is known and that we are not scalarising the vector.

I have added tests here that show we correctly return an invalid cost
for VF=vscale x 1 when the loop contains unusual types such as i7:

  Transforms/LoopVectorize/AArch64/sve-inductions-unusual-types.ll


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D113772

Files:
  llvm/include/llvm/Analysis/TargetTransformInfoImpl.h
  llvm/include/llvm/CodeGen/BasicTTIImpl.h
  llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
  llvm/test/Transforms/LoopVectorize/AArch64/sve-inductions-unusual-types.ll

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