No optimisation currently done in production compilers (that I know of) work on more than a few blocks. I haven't seen anyone claim they're not general purpose or don't scale yet, or are not reliable.

Meanwhile the patch we're discussing is 2 functions, 3 blocks each. So in this context we're not talking about anything crazy.

Examples of optimization passes in llvm that reason across arbitrary numbers of blocks:

- SROA

- GVN

- regalloc

And those are just the heaviest hitters. In fact most optimization passes are capable of performing changes that impact many blocks at once.

Eh, kinda? I mean regalloc is not an optimisation pass. And SROA/GVM go through whole functions, but they don't really optimise across blocks. They do local changes only with information from previous blocks available. I meant that I don't know of anything that makes modifications across multiple blocks as one complex change. Even licm pokes one thing at a time from one block to a different one. There's no complex "let's replace this kind of cross-block pattern with a different equivalent code that changes multiple blocks at the same time".

SROA absolutely does a coordinated modification across an arbitrary number of blocks whenever it kills an alloca.

GVN has cases where it’ll also make a coordinated change across multiple blocks

And those are just two passes. I’m not mentioning the many other passes that have this property just for the sake of brevity.