Use getMostFrequentByte to decide if should used memset+stores

[vitalybuka]

Summary

Previous implementation handled only memsets to zero.
This patch keeps behavior as close as possible to the original.
In followup patches I'll resolve TODOs and update/add tests.

Significantly changed part of @pcc prototype for -ftrivial-auto-var-init=pattern optimizations

[vitalybuka]

Reverted debug logging

[jfb]

clang/lib/CodeGen/CGDecl.cpp

865

You should update the return value to Optional<Value*> where the non-None return is the bytewise value.

873

I'm not sure I understand what this bit does. If something isn't a bytewise value then we now automatically assume it can be stored to with a single store. That's not necessarily true. I think you need to drop BWInit || from here, and conditionalize this entire branch with if (BWInit) instead (so, an && on it all).

888

The recursion now allows different values for BWInit on each sub-element.

895

What does this new loop do? It'll only ever go around the loop once.

[vitalybuka]

undo some behavior change

[vitalybuka]

rebase

[vitalybuka]

clang/lib/CodeGen/CGDecl.cpp

865

Why?
We already know value which was used for memset, now we just counting additional store/memsets

873

If value is bytewise, we will emit BWInit

888

yes

895

it counts number of stores used by elements
similar look we had in canEmitInitWithFewStoresAfterBZero

[vitalybuka]

clang/lib/CodeGen/CGDecl.cpp

873

If value is bytewise, we will emit BWInit

If value is bytewise, we will emit memset, which we can count as one store.
It's not always true that memset costs as a single store. However this is a heuristic, as the limit 6 itself.

Here I try to keep behavior close to original for zeroes.
In future patches I'd like to try to remove this function and use just "density" ((count memset bytes that match initializer)/(full size of initializer)). This will be simpler, should work good enough for exiting cases, and cover new cases with large initializers where 6 is not enough,