[Speed] scope.Findvar maybe slow down all the operators

[typhoonzero]

Here's some reference I've searched:
https://softwareengineering.stackexchange.com/questions/49550/which-hashing-algorithm-is-best-for-uniqueness-and-speed

Every operator need to call Findvar directly or indirectly.

As C++ default hash<std::string> implementation uses "murmurhash", which is the fastest hash function, it may still slower than directly lookup a variable using an offset. We can consider a way to make runtime Findvar implementation do not use unordered_map.

[helinwang]

Thanks for the issue!
Maybe the hash table lookup cost is marginal, comparing to the time it take for running GPU kernels? I think we need to have some profiling numbers to back this up.

[tonyyang-svail]

@typhoonzero Any follow-ups on this issue? The hash lookup usually takes 200ns. The gap between Op.Run() is usually 500,000 ~ 1,000,000 ns. I doubt the loop up time is the issue.

[dzhwinter]

One month ago, I had discussed this issue with @typhoonzero, we thought that it will not degrade the performance magnitude. Because 200ns means 200 * 10-12, it is really small.
But to give a solid conclusion, now I'm doing an experiment on this changing the unordered_map to index based vector, will report the result later.

[typhoonzero]

Closing this for now!