[llvm-dev] [RFC] Add TargetTransformInfo::isAllocaPtrValueNonZero and let ValueTracking depend on TargetTransformInfo

[Liu, Yaxun (Sam) via llvm-dev]

Some optimizations depend on whether alloca instruction always has non-zero value. Currently, this checking is done by isKnownNonZero() in ValueTracking, and it assumes alloca in address space 0 always has non-zero value but alloca in non-zero address spaces does not always have non-zero value.

However, this assumption is incorrect for certain targets. For example, amdgcn---amdgiz target has alloca in address space 5, and its alloca always has non-zero value. This assumption causes some optimizations disabled for amdgcn---amdgiz target.

After discussions at https://reviews.llvm.org/D40670, I propose to introduce TargetTransformInfo::isAllocaPtrValueNonZero for representing whether alloca instruction always has non-zero value, and add a TargetTransformInfo argument to ValueTracking functions e.g. isKnownNonZero().

As a result, passes using ValueTracking will require TargetTransformInfo.

Comments are welcome.

Thanks.

Sam
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