[llvm] [llvm-cov] Simplify and optimize MC/DC computation (PR #79727)

[NAKAMURA Takumi via llvm-commits]

chapuni wrote:

@evodius96

> You mean construct the list of possible test vectors in clang?

Planning let `clangCodeGen` emit `TestVectorIndex` w/o constructing square vectors.

> I could understand perhaps extending beyond six, but why thousands? Achieving full MC/DC is not easy even with six, and most embedded cases would have no more than maybe 3 or 4 conditions. I'm concerned about overengineering.

"Thousands" is not my goal but my expected capacity. Not measured yet.
I think satisfying MC/DC Coverage would be the pain but not punishment, though. :) I also think MC/DC Coverage is one of useful method to discover missing cases.

I am doing it since it is very effective to apply it to the large codebase, LLVM itself, since we could keep it stable and preventing bitrot. I like such a dogfooding. :)

Thank you.

https://github.com/llvm/llvm-project/pull/79727