LSRA: Heuristics for callee saved reg allocation.

[sivarv]

LSRA allocates callee saved register to the first var (preferenced to a callee saved reg) that is encountered in linear order. Later when a more deserving var is found needing a callee saved register LSRA won't take it back from a less deserving candidate.

Put another way, improve heuristic to decided when LSRA should not allocate at all to a var preferenced to a callee saved reg.

Specifically LSRA is conservative with fp vars.

Example benchmarks:
SciMark FFT
Huffman
MemoryPoolIterator:CopyFrom()

category:cq
theme:register-allocator
skill-level:expert
cost:medium

[kunalspathak]

We want to tune our overall heuristics based on the experiments we do as part of #43318. While we will do the experiments in .NET 6, it is highly unlikely that we would bake that in produce during .NET 6. Moving to future.

[kunalspathak]

Currently, we do track local vars whose refCntWtd meets the threshold.

runtime/src/coreclr/jit/lsra.cpp

Lines 1837 to 1840 in 8d66372

	if (refCntWtd >= thresholdFPRefCntWtd)
	{
	VarSetOps::AddElemD(compiler, fpCalleeSaveCandidateVars, varDsc->lvVarIndex);
	}

Later, when LSRA allocates registers, we never go back and reset the assignment if any other variable deserves that register more.