Re: [PATCH v2 2/3] arm64: Optimize __READ_ONCE() with CONFIG_LTO=y

[David Laight <david.laight.linux@gmail.com>]

On Thu, 29 Jan 2026 01:52:33 +0100
Marco Elver <elver@google.com> wrote:

> Rework arm64 LTO __READ_ONCE() to improve code generation as follows:
> 
> 1. Replace _Generic-based __unqual_scalar_typeof() with more complete
>    __rwonce_typeof_unqual(). This strips qualifiers from all types, not
>    just integer types, which is required to be able to assign (must be
>    non-const) to __u.__val in the non-atomic case (required for #2).
> 
> Once our minimum compiler versions are bumped, this just becomes
> TYPEOF_UNQUAL() (or typeof_unqual() should we decide to adopt C23
> naming).  Sadly the fallback version of __rwonce_typeof_unqual() cannot
> be used as a general TYPEOF_UNQUAL() fallback (see code comments).
> 
> One subtle point here is that non-integer types of __val could be const
> or volatile within the union with the old __unqual_scalar_typeof(), if
> the passed variable is const or volatile. This would then result in a
> forced load from the stack if __u.__val is volatile; in the case of
> const, it does look odd if the underlying storage changes, but the
> compiler is told said member is "const" -- it smells like UB.
> 
> 2. Eliminate the atomic flag and ternary conditional expression. Move
>    the fallback volatile load into the default case of the switch,
>    ensuring __u is unconditionally initialized across all paths.
>    The statement expression now unconditionally returns __u.__val.
> 
...
> Signed-off-by: Marco Elver <elver@google.com>
> ---
> v2:
> * Add __rwonce_typeof_unqual() as fallback for old compilers.
> ---
>  arch/arm64/include/asm/rwonce.h | 24 ++++++++++++++++++++----
>  1 file changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
> index fc0fb42b0b64..712de3238f9a 100644
> --- a/arch/arm64/include/asm/rwonce.h
> +++ b/arch/arm64/include/asm/rwonce.h
> @@ -19,6 +19,23 @@
>  		"ldapr"	#sfx "\t" #regs,				\
>  	ARM64_HAS_LDAPR)
>  
> +#ifdef USE_TYPEOF_UNQUAL
> +#define __rwonce_typeof_unqual(x) TYPEOF_UNQUAL(x)
> +#else
> +/*
> + * Fallback for older compilers to infer an unqualified type.
> + *
> + * Uses the fact that auto is supposed to drop qualifiers. Unlike

Maybe:
	In all versions of clang 'auto' correctly drops qualifiers.
A reminder in here that this is clang only might also clarify things.

> + * typeof_unqual(), the type must be complete (defines an unevaluated local
> + * variable); this must trivially hold because __READ_ONCE() returns a value.

Not sure that is needed.

> + *
> + * Another caveat is that because of array-to-pointer decay, an array is
> + * inferred as a pointer type; this is fine for __READ_ONCE usage, but is
> + * unsuitable as a general fallback implementation for TYPEOF_UNQUAL.

gcc < 11.0 stops it being used elsewhere.
Something shorter?
	The arrary-to-pointer decay doesn't matter here.

  David

> + */
> +#define __rwonce_typeof_unqual(x) typeof(({ auto ____t = (x); ____t; }))
> +#endif
> +
>  /*
>   * When building with LTO, there is an increased risk of the compiler
>   * converting an address dependency headed by a READ_ONCE() invocation
> @@ -32,8 +49,7 @@
>  #define __READ_ONCE(x)							\
>  ({									\
>  	typeof(&(x)) __x = &(x);					\
> -	int atomic = 1;							\
> -	union { __unqual_scalar_typeof(*__x) __val; char __c[1]; } __u;	\
> +	union { __rwonce_typeof_unqual(*__x) __val; char __c[1]; } __u;	\
>  	switch (sizeof(x)) {						\
>  	case 1:								\
>  		asm volatile(__LOAD_RCPC(b, %w0, %1)			\
> @@ -56,9 +72,9 @@
>  			: "Q" (*__x) : "memory");			\
>  		break;							\
>  	default:							\
> -		atomic = 0;						\
> +		__u.__val = *(volatile typeof(*__x) *)__x;		\
>  	}								\
> -	atomic ? (typeof(*__x))__u.__val : (*(volatile typeof(*__x) *)__x);\
> +	__u.__val;							\
>  })
>  
>  #endif	/* !BUILD_VDSO */