Re: More annoying code generation by clang

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 04, 2024 at 03:53:49PM -0700, Linus Torvalds wrote:

> diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
> index 66e57c010392..6159d2cbbfde 100644
> --- a/arch/x86/include/asm/barrier.h
> +++ b/arch/x86/include/asm/barrier.h
> @@ -33,20 +33,15 @@
>   * Returns:
>   *     0 - (index < size)
>   */
> +#define array_index_mask_nospec(idx,sz) ({	\
> +	typeof((idx)+(sz)) __idx = (idx);	\
> +	typeof(__idx) __sz = (sz);		\
> +	typeof(__idx) __mask;			\
> +	asm volatile ("cmp %1,%2; sbb %0,%0"	\
> +			:"=r" (__mask)		\
> +			:"ir"(__sz),"r" (__idx)	\
> +			:"cc");			\
> +	__mask; })

Should this not carry a comment about the "ir" constraint wanting to be
"g" except for clang being daft?

(I really wish clang would go fix this, it keeps coming up time and
again).

>  
>  /* Prevent speculative execution past this barrier. */
>  #define barrier_nospec() asm volatile("lfence":::"memory")