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1. For a constant where the high and low halves only differ by a few bits, consider using pack as an intermediate step and the Zbs instructions to make final adjustments.
2. When possible derive the upper and lower halves from a common sequence, so for example sharing a lui
3. Similarly for cases where the high half can be constructed by merely shifting the low half.
4. Improve handling of constants where the upper and lower 32bit halves are the same and zbkb is not available.  It is believed this is a relatively small adjustment to the existing code.
5. Reevaluate the mvconst_internal pattern.   There's some belief that it could be eliminated or used much less often.  Right now it "lies" to the combiner and indicates it can load an arbitrary 64bit constant as a single instruction.  That's a useful property when we can to combine the constant in another instruction, but it's painful in other ways such as its interaction with the code to improve additional of constants that are not simm12, but are 2*simm12.
6. Some constants may be generated with something like lui+shadd+addi where the lui+shadd targets a nearby constant and the final addi fixes things up.  The approach of targeting a nearby constant with a fixup at the end probably applies to most synthesis opportunities where the cost is 4+ instructions.

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