JIT: Eliminate RegTracker

[CarolEidt]

Now that LEGACY_BACKEND has been eliminated, the RegTracker is not longer useful. It was used by the legacy backend to avoid reloading local variables or setting a register to a constant if it was already available in a register. Some constant reuse has been implemented in the register allocator, and future work to support reuse of (memory-based) locals or constants would be done elsewhere (e.g. see issue #6264).

Currently the RegTracker methods only call regSet->rsSetRegsModified(), however the register allocator should already be doing this for any registers it allocates, as well as any registers killed by an instruction. Furthermore, the code generator should already be doing this for any additional registers used, for example, in prolog or epilog code.

Calls to these methods should be replaced with methods on RegSet that simply verify that the register is in the modified set, and assert otherwise (and would be a no-op in retail builds), e.g.:

• RegSet::verifyRegUsed(regNumber reg)

  • this would replace calls to RegTracker::rsTrackRegTrash(), RegTracker::rsTrackRegIntCns(), RegTracker::rsTrackRegLclVar() and RegTracker::rsTrackRegCopy().

• RegSet::verifyRegistersUsed(regMaskTP regMask)

  • this would replace calls to RegTracker::rsTrashRegSet()

[CarolEidt]

This issue would be a good "getting started" project for someone wanting to gain familiarity with the JIT. It would be good to build coreclr on your chosen target, and to read the JIT overview and tutorial documents: https://github.com/dotnet/coreclr/blob/master/Documentation/botr/ryujit-overview.md and https://github.com/dotnet/coreclr/blob/master/Documentation/botr/ryujit-tutorial.md

[CarolEidt]

Closing this, and transfering comments to #10411