linux-kernel - Re: [PATCH 3/5] rust: sync: support using bool with READ

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Message-ID: <20260107083327.GB272712@noisy.programming.kicks-ass.net>
Date: Wed, 7 Jan 2026 09:33:27 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Gary Guo <gary@...yguo.net>
Cc: Alice Ryhl <aliceryhl@...gle.com>, Boqun Feng <boqun.feng@...il.com>,
	Will Deacon <will@...nel.org>,
	Richard Henderson <richard.henderson@...aro.org>,
	Matt Turner <mattst88@...il.com>,
	Magnus Lindholm <linmag7@...il.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	FUJITA Tomonori <fujita.tomonori@...il.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Lyude Paul <lyude@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	John Stultz <jstultz@...gle.com>, Stephen Boyd <sboyd@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 3/5] rust: sync: support using bool with READ_ONCE

On Tue, Jan 06, 2026 at 06:12:01PM +0000, Gary Guo wrote:
> On Tue, 6 Jan 2026 13:43:26 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:

> > Does this hardcode that sizeof(_Bool) == 1? There are ABIs where this is
> > not the case.
> 
> Hi Peter,
> 
> Do you have a concrete example on which ABI/arch this is not true?
> 
> I know that the C spec doesn't mandate _Bool and char are of the same size
> but we have tons of assumptions that is not guaranteed by standard C..

Darwin/PowerPC famously has sizeof(_Bool) == 4

Win32: Visual C++ 4.2 (and earlier) had sizeof(bool)==4 (they mapped
bool to int), while Visual C++ 5.0 introduced a native _Bool and moved
to 1 byte.

Early RISC CPUs (MIPS, PowerPC, Alpha) had severe penalties for byte
access and their compilers would've had sizeof(bool)=={4,8}.

I think AVR/Arduino also has sizeof(bool) == sizeof(int) which is 2.