I think the C extension should be targeted by default for RV64. i.e. RV64GC (RV64IMAFDC). Check with the RISC-V Foundation. C is now enabled by default in the RV64 toolchain.
BTW shipping silicon is RV32IMAC, which is why I mentioned it, however this is not really a mainstream distribution target as it's embedded. There will be RV64GC hardware soonish I hope.
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On 20/04/2017, at 1:41 AM, Luke Kenneth Casson Leighton <email@example.com> wrote: