Re: [PATCH v4 05/10] x86: Add functions for 64-bit integer arithmetic

From: "Jan Beulich" <JBeulich@suse.com>
To: Haozhong Zhang <haozhong.zhang@intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>, Keir Fraser <keir@xen.org>,
	Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	xen-devel@lists.xen.org,
	Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>,
	Jun Nakajima <jun.nakajima@intel.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH v4 05/10] x86: Add functions for 64-bit integer arithmetic
Date: Fri, 05 Feb 2016 06:36:55 -0700	[thread overview]
Message-ID: <56B4B38702000078000CF09F@prv-mh.provo.novell.com> (raw)
In-Reply-To: <1453067939-9121-6-git-send-email-haozhong.zhang@intel.com>

>>> On 17.01.16 at 22:58, <haozhong.zhang@intel.com> wrote:
> This patch adds several functions to take multiplication, division and
> shifting involving 64-bit integers.
> 
> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> ---
> Changes in v4:
>  (addressing Jan Beulich's comments)
>  * Rewrite mul_u64_u64_shr() in assembly.

Thanks, but it puzzles me that the other one didn't get converted
as well. Anyway, I'm not going to make this a requirement, since
at least it appears to match Linux'es variant.

> +static inline u64 mul_u64_u64_shr(u64 a, u64 mul, unsigned int n)
> +{
> +    u64 hi, lo;
> +
> +    asm volatile ( "mulq %2; shrdq %1,%0"
> +                   : "=a" (lo), "=d" (hi)
> +                   : "rm" (mul), "0" (a), "c" (n) );

SHRD formally is a 3-operand instruction, and the fact that gas'
AT&T syntax supports a 2-operand "alias" is, well, odd. Please
let's use the specification mandated 3-operand form properly,
to avoid surprises with e.g. clang.

Jan