linux-kernel - Re: More annoying code generation by clang

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Message-ID: <20240408084934.GC21904@noisy.programming.kicks-ass.net>
Date: Mon, 8 Apr 2024 10:49:34 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Ingo Molnar <mingo@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Peter Anvin <hpa@...or.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: More annoying code generation by clang

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")