Before this patch, CGLoop would dump all transformations for a loop into a single LoopID without encoding any order in which to apply them. rL348944 added the possibility to encode a transformation order using followup-attributes.
When a loop has more than one transformation, use the follow-up attribute define the order in which they are applied. The emitted order is the defacto order as defined by the current LLVM pass pipeline, which is:
This patch should therefore not change the assembly output, assuming that all explicit transformations can be applied, and no implicit transformations in-between. In the former case, WarnMissedTransformationsPass should emit a warning (except for MachinePipeliner which is not implemented yet). The latter could be avoided by adding 'llvm.loop.disable_nonforced' attributes.
Because LoopUnrollAndJamPass processes a loop nest, generation of the MDNode is delayed to after the inner loop metadata have been processed. A temporary LoopID is therefore used to annotate instructions and RAUW'ed by the actual LoopID later.