Hacker News Reader

This is quite interesting. Is there anything preventing the collection of the number of cycles spent in an arbitrary function? It seems this is just a matter of identifying all branches.

View on HN

Yes, that might be possible. However, you probably will get multiple cycle counts for the same function depending on which path was taken. And it works only if the amount of taken branches in the function is not that big (less than 32). Otherwise it will not fit into LBR stack. For example, if you have a loop with more than 32 iterations it will trash the LBR stack with backwards jumps. But yeah, for small functions it might work pretty well.

I would better go for analyzing not the whole function (all basic blocks of the function) but only the Hyper Blocks (typical hot path through the function). Here is the example of how to do it: https://lwn.net/Articles/680985/ chapter "Hot-path analysis".

Superblocks rather than Hyperblocks. Except for cmov which is partial predication, x86 doesn’t have predication. But SBs are probably what your optimizer wants anyways.

Reply on news.ycombinator.com