[clang] [llvm] [RISCV] Add -mcpu=sifive-p550. (PR #122164)

[Craig Topper via llvm-commits]

topperc wrote:

> I thought the P550 was supposed to support RV64GCB, but that would require Zbs support, which isn't listed here.

The P550 version in the HiFive Premier board does not support Zbs. ISA string from /proc/cpuinfo is `rv64imafdch_zicsr_zifencei_zba_zbb_sscofpmf`

https://github.com/llvm/llvm-project/pull/122164