This patch introduces a new infrastructure that is used to select the load and store instructions in the PPC backend.
The primary motivation is that the current implementation of selecting load/stores is dependent on the ordering of patterns in TableGen.
Given this limitation, we are not able to easily and reliably generate the P10 prefixed load and stores instructions (such as when
the immediates that fit within 34-bits). This refactoring is meant to provide us with more control over the patterns/different forms to exploit,
as well as eliminating dependency of pattern declaration in TableGen.
The idea of this refactoring is that it introduces a set of addressing modes that correspond to different instruction formats
of a particular load and store instruction, along with a set of common flags that describes a load/store. Whenever a load/store
instruction is being selected, we analyze the instruction and compute a set of flags for it. The computed flags are then used to
select the most optimal load/store addressing mode.
This computation of flags is done in computeMOFlags(), while selecting the optimal addressing mode is done through
getAddrModeForFlags(); in which this functions searches for a set of address flags stored in a map that relates common flags to
addressing modes. Once the optimal addressing mode is determined, this information is given to SelectOptimalAddrMode(),
where we set the base and displacement of the load/store accordingly based on the addressing mode.
Another thing to note is the SelectForceXForm() function is similar to SelectAddressRegRegOnly(), with an updated naming
and a removed condition to better suit the refactoring that is being done on the loads/stores.
This patch is the first of a series of patches to be committed - it contains the initial implementation of the refactored load/store
selection infrastructure and also updates P8/P9 patterns to adopt this infrastructure. The idea is that incremental patches will
add more implementation and support, and eventually the old implementation will be removed.