linux-kernel - Re: [PATCH v2 01/10] riscv: Implement cmpxchg32/64() using Zacas

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Message-ID: <Zo3MH8idihW4o+6Z@andrea>
Date: Wed, 10 Jul 2024 01:47:43 +0200
From: Andrea Parri <parri.andrea@...il.com>
To: Alexandre Ghiti <alex@...ti.fr>
Cc: Alexandre Ghiti <alexghiti@...osinc.com>,
	Jonathan Corbet <corbet@....net>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Nathan Chancellor <nathan@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>,
	Arnd Bergmann <arnd@...db.de>, Leonardo Bras <leobras@...hat.com>,
	Guo Ren <guoren@...nel.org>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH v2 01/10] riscv: Implement cmpxchg32/64() using Zacas

> > Is this second IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) check actually needed?
> > (just wondering - no real objection)
> 
> To me yes, otherwise a toolchain without zacas support would fail to
> assemble the amocas instruction.

To elaborate on my question:  Such a toolchain may be able to recognize
that the block of code following the zacas: label (and comprising the
amocas instruction) can't be reached/executed if the first IS_ENABLED()
evaluates to false (due to the goto end; statement), and consequently it
may compile out the entire block/instruction no matter the presence or
not of the second IS_ENABLE() check.  IOW, such a toolchain/compiler may
not actually have to assemble the amocas instruction under such config.
In fact, this is how the current gcc trunk (which doesn't support zacas)
seems to behave.  And this very same optimization/code removal seems to
be performed by clang when CONFIG_RISCV_ISA_ZACAS=n.  IAC, I'd agree it
is good to be explicit in the sources and keep both of these checks.

> > Why the semicolon?
> 
> That fixes a clang warning reported by Nathan here:
> https://lore.kernel.org/linux-riscv/20240528193110.GA2196855@thelio-3990X/

I see.  Thanks for the pointer.

> > This is because the compiler doesn't realize __ret is actually
> > initialized, right?  IAC, seems a bit unexpected to initialize
> > with (old) (which indicates SUCCESS of the CMPXCHG operation);
> > how about using (new) for the initialization of __ret instead?
> > would (new) still work for you?
> 
> But amocas rd register must contain the expected old value in order to
> actually work right?

Agreed.  Thanks for the clarification.

  Andrea