[PATCH] D128005: [SelectionDAG] Don't apply MinRCSize constraint in InstrEmitter::AddRegisterOperand for IMPLICIT_DEF sources.

[Philip Reames via Phabricator via llvm-commits]

reames added a comment.

In D128005#3590328 <https://reviews.llvm.org/D128005#3590328>, @arsenm wrote:

> I've never understood the point of this parameter

I went digging a bit, and as best I can tell this is a heuristic to avoid putting too much pressure on register allocation for virtual regs from small register classes.  The idea appears to be that using values copied into a bigger register class is likely to lead to overall better results.  As a guess, I think this is a workaround for the fact that we can't spill to other register classes.

For the implicit_def case, that heuristic makes no sense.  So LGTM by me too.


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