[LSRA][RyuJIT] Add support to allow either of op1 or op2 to be considered as reg optional rather than Lower fixing one of the operands as reg optional #6358

Right now reg-optional operand support requires Lower marking a specific operand as reg-optional operand. Lower cannot know apriori which operand is not going to get a register and can only make a guess using heuristics. Analysis indicates, there are quite a few cases which are missed as a result.

Rather than fixing a particular operand as reg-optional it would be advantageous if Lower just specified the max number of operands that could be considered as reg-optional on the parent node and marking all operands that are permissible to be reg-optional.

Example1:
consider GT_ADD(op1, op2).
GT_ADD:
srcRegOptionalCount = 1
Mark both op1 and op2 as reg-optional

Example2:
Consider GT_MUL(op1, containable immed)
Code generated is: imul TargetReg, [mem], immed

Here op1 can be reg optional.
GT_MUL:
srcRegOptionalCount = 1
mark op1 as reg optional.

LSRA needs to ensure while allocating no more than one operand of GT_ADD is treated as reg-optional and hence will be considered as contained memory operand.

Some initial thoughts on how to implement this:
Each ref position now also carries with it a ptr to GenTree where it is consumed.
LSRA while creating def position for GT_ADD will update use positions of op1 and op2 to refer to GT_ADD.
During reg allocation, LSRA will decrement srcRegOptionalCount whenever any of its reg optional operands at use is not allocated a reg. LSRA will not consider a use position as reg option if srcRegOptionalCount of its parent node becomes zero.

It is possible that a reg is allocated to both op1 and op2 since free registers were available. Later one of op1 or op2 could be selected for spilling. allocateBusyReg() also needs to be modified to make sure we don't end up having both the operands in memory - particularly for tree temps because tracked lcls are always marked spill after.

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