golang
cmd/compile/internal/ssa: add codegen for Zicond extension on riscv64
Zicond is a mandatory extension in rva23u64. This patch converts branches to CondSelect and optimizes them to Zicond instructions on RISC-V, along with additional optimization rules.
Zicond can provide performance benefits on unpredictable branches by avoiding branch misprediction penalties. However, on simple predictable branches, zicond uses 4 instructions vs 2 for traditional branches, which can cause performance regressions.
To avoid regressions, we keep CondSelect globally disabled for riscv64 and only enable it for the ConstantTimeSelect intrinsic, which has been shown to benefit from zicond:
goos: linux goarch: riscv64 pkg: crypto/subtle CPU: SG2044 │ nozicond.txt │ zicond.txt │ │ sec/op │ sec/op vs base │ ConstantTimeSelect-44 2.325n ± 4% 1.750n ± 2% -24.69% (p=0.000 n=10)
Future work can explore enabling zicond for other cases that can benefit from zicond.
Follow-up to CL 631595 Updates #75350
Co-authored-by: [email protected] [email protected]
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