[cortesoft]

What is the cost of this pollution?

I am genuinely curious, I don't know much about instruction set design.

[freeone3000]

This uses existing rounding modes with pre-set flags, so it costs 1 entry in the LUT and a small number of muxes, one per flag, assuming the worst case.

[Stevvo]

I guess it's however many bytes the instruction occupys in L1 cache.

[saagarjha]

The pollution would be in the chip complexity.