[llvm] [llvm][ARM] Add a cortex-m4f alignment hazard recognizer (PR #126991)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Hi - High level comment, on the assumption that a nop costs 1 cycle and takes 2 bytes, does this mean that:
 - 1 T2 instruction ends up at -1 cycle, 2 extra bytes.
 - 2 T2 instructions is the same, -1 cycle, 2 extra bytes.
 - 3 T3 instructions is now 0 cycles, 2bytes.
 - 4 and 5 are the same as 3.
 - 6 is 1 cycle better, 2 bytes spent. Better improvement for longer chains.

If so does this have a fairly high chance of increasing runtime? Would it be better to look across the inner loop BB and check if adding a nop will save more than it helps? Or does it not work like that? Thanks.

https://github.com/llvm/llvm-project/pull/126991