cmd/compile/internal/ssa: add codegen for Zicond extension on riscv64 #75577

lxq015 · 2025-09-24T04:38:54Z

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: wangpengcheng.pp@bytedance.com
mengzhuo1203@gmail.com

gopherbot · 2025-09-24T04:48:07Z

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gopherbot · 2025-09-24T04:55:52Z

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gopherbot · 2025-09-24T09:34:03Z

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gopherbot · 2025-09-25T08:45:22Z

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gopherbot · 2025-09-25T09:09:46Z

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gopherbot · 2025-10-13T14:33:02Z

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gopherbot · 2025-10-19T21:33:31Z

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gopherbot · 2025-10-21T08:27:32Z

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lxq015 force-pushed the zicond-codegen-support branch from 2f5ba88 to 58e1e0c Compare September 25, 2025 08:44

lxq015 force-pushed the zicond-codegen-support branch from 9e3f91a to cd67025 Compare November 4, 2025 07:04

lxq015 added 17 commits December 3, 2025 17:38

fix test case

fe82c24

add some rules

e834c58

fix merge conflict and form of rules

249f436

use spaces instead of commas in codegen tests

09ec722

fix bug

3235a05

use MOVBUreg to zero the upper bytes

d6e0dcd

delete extra space

aaf68f3

fix rule

d33a4c1

zicond_performance

34895f8

zicond-refine

3e8bd27

delete useless file

e267694

fix branchelim.go

10a87ed

refine condselect cost

77ed2f2

delete useless tests

f86d75a

Remove redundant comments

43a63d6

fix funcname

3c5818d

keep condselect disabled

29e37a3

lxq015 force-pushed the zicond-codegen-support branch from f87591b to 29e37a3 Compare December 3, 2025 09:38

cmd/compile: add ConstantTimeSelect intrinsics for riscv64

7a61508

cmd/compile/internal/ssa: add codegen for Zicond extension on riscv64 #75577

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cmd/compile/internal/ssa: add codegen for Zicond extension on riscv64 #75577

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