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Message-ID: <d6789d7c-f58c-49b1-9677-b922266fb80d@isrc.iscas.ac.cn>
Date: Thu, 16 Oct 2025 23:37:36 +0800
From: Pincheng Wang <pincheng.plct@...c.iscas.ac.cn>
To: musl@...ts.openwall.com
Subject: Re: [PATCH 0/1] riscv: Add support for Zacas in atomic
operations
On 2025/9/19 00:47, Pincheng Wang wrote:
> Hi all,
>
> This patch adds support for the RISC-V Zacas (Atomic Compare-and-Swap)
> extension in musl's atomic operations for both riscv64 and riscv32.
>
> Currently, musl implements a_cas using a
> Load-Reserved/Store-Conditional (lr/sc) loop that:
> - Requires at least four instructions (lr+bne+sc+bnez) per CAS
> operation,
> - Contains a retry loop under contention,
> - Incurs branch penalties that may cause pipeline stalls.
>
> Zacas introduces amocas.w.aqrl/amocas.d.aqrl instructions that perform
> CAS atomically in a single instruction, eliminating retry loops and
> conditional branches.
>
> Due to hardware limitations, we evaluated this change under QEMU using
> both mcycle and minstret counters. The results show clear benefits:
>
> Metric lr/sc Zacas Improvement
> Instr. per CAS (50k ops average) 15.04 8.36 -44.4%
> Instr. per op (single-thread) 23.61 14.25 -39.6%
> Instr. per op (multi-thread, high contention) 528.24 251.14 -52.5%
>
> In addition, libc.a size is reduced by ~1.2% due to removal of loop
> code.
>
> The patch automatically falls back to the lr/sc implementation on
> systems where Zacas is not available, preserving full backward
> compatibility.
>
> This work provides a measurable reduction in instruction count,
> execution cycles and binary size, improving scalability of
> synchronization primitives under load.
>
> Thanks for reviewing!
>
> Best regards,
> Pincheng Wang
>
>
> Pincheng Wang (1):
> riscv: add Zacas extension support for atomic CAS
>
> arch/riscv32/atomic_arch.h | 17 +++++++++++++++++
> arch/riscv64/atomic_arch.h | 30 ++++++++++++++++++++++++++++++
> 2 files changed, 47 insertions(+)
>
Hi all,
Friendly ping regarding my earlier patch on enabling the RISC-V Zacas
(amocas.{w,d}) path for a_cas()/a_cas_p().
Best regards,
Pincheng Wang
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