Introduce local variables with unknown size

[snickolls-arm]

Adds ValueSize class for handling sizes that may be unknown at compile time during phases such as value numbering, that aren't concerned with the exact numeric value (only properties such as equivalence etc.). Adds various assertions to areas of value numbering and SSA building with assumptions based on exact size values. Adds LclVarDsc::lvValueSize to produce a ValueSize for a local variable, which provides the criteria for which locals have unknown size at compile time.

[snickolls-arm]

@dotnet/arm64-contrib

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[snickolls-arm]

@dotnet/arm64-contrib @dotnet/jit-contrib Please could I have a review for this patch? It's a precursor for supporting the rest of the local variable implementation for SVE Vector<T>, which I can also send if more context is needed.

[jakobbotsch]

What prevents a normal struct from having size UINT8_MAX?

[EgorBo]

genTypeSizes for TYP_STRUCT returns 0 so it's fine?

[EgorBo]

there is already TYP_UNKNOWN - can it be re-used here?

[jakobbotsch]

genTypeSizes for TYP_STRUCT returns 0 so it's fine?

LclVarDsc::lvExactSize will assert for structs of size 255 in this PR if I am understanding correctly.

I think having a SIZE_UNKNOWN returned by lvSymbolicSize that you can forget to check is problematic and fragile. For example, it leaks into all users of VisitLocalDefs without good confidence that everyone remembers to check for it. I would much rather have this be some kind of optional value that makes it clear when it may not be present.

[snickolls-arm]

I would much rather have this be some kind of optional value that makes it clear when it may not be present.

Say we have unsigned lvExactSize(bool *hasUnknownSize) instead? And I'll update all the call sites to decide whether hasUnknownSize is relevant or not to the situation.

[jakobbotsch]

No, I would rather have a struct like

struct ValueSize
{
  bool IsUnknown();
  unsigned GetSize()
  {
    assert(!IsUnknown());
    return ...;
  }
};

to be used in the few places where we want to propagate a possibly unknown size, like VisitLocalDefs. Its internal representation can represent unknown as UINT_MAX (I don't think UINT8_MAX is reasonable).

But I would keep lvExactSize as asserting that it is not called on a value of unknown size. Hopefully there won't be many places where we need to even deal with something like ValueSize.

[snickolls-arm]

Hopefully there won't be many places where we need to even deal with something like ValueSize.

It seems to be limited to value numbering, and after implementing your suggestion I can see which areas have the potential to fire assertions more clearly. I've updated the description to reflect this better.