[llvm] [RISCV] Add short forward branch support for `min`, `max`, `maxu` and `minu` (PR #164394)

[via llvm-commits]

hchandel wrote:

> > > > SiFive cores do not support short forward branch for MIN/MAX.
> > > 
> > > 
> > > We did it this way to avoid introducing tune features that depend on other tune features, but I see there are more instruction differences than I expected. I hoped that SiFive cores might not have Zbb when they don't do this fusion, but I should have checked closer.
> > > Our plan is to add another feature to enable these SFB cases, which would be required. I was originally thinking of naming it `TuneShortForwardBranchOptZbb`, but I see there are already Zbb instructions supported by SFB that you presumably don't want disabled if you don't support SFB for min/max (SFB for ANDN, ORN, XNOR).
> > > The hypothetical `TuneShortForwardBranchOptZbb` would require/imply `TuneShortForwardBranchOpt`, and the former would be required to SFB min/max.
> > > The idea behind this naming scheme is it should be obvious for extending to other extensions, as I think we will also want it for a `TuneShortForwardBranchOptZmmul` (or similar), as you've said you don't fuse branches with MUL.
> > > What are your thoughts?
> > > I've asked Harsh to prepare this patch update (as well as other updates), but it would be good to hear you like this direction before we upload the next version.
> > 
> > 
> > I'm not sure what a good name is. The instructions that aren't supported are the ones that are only available on PipeB in RISCVSchedSiFive7.td. That includes div/rem, mul, ctz, ctz, ctpop, rotate, shXadd, orc.b, bset(i), bclr(i), binv(i).
> > Maybe we should rename the existing flag to TuneShortForwardBranchOptIALU? IALU being the scheduler class that covers the supported instructions?
> 
> Yeah, let's come back to what to rename the existing option later, but it does seem like these instructions (min/max) are not in the IALU scheduling class, so let's intend to name them `TuneShortForwardBranchSCHEDCLASS` - for this one it would be `TuneShortForwardBranchIMinMax`

Done. Please have a look at the latest changes.

https://github.com/llvm/llvm-project/pull/164394