linux-kernel - Re: [PATCH 1/2] math128: Introduce {mult,add,cmp}

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Date:	Wed, 25 Apr 2012 15:29:36 +0100
From:	Jay Foad <jay.foad@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/2] math128: Introduce {mult,add,cmp}_u128

> +#ifndef shl_u128
> +static inline u128 shl_u128(u128 x, unsigned int n)
> +{
> +	u128 res;
> +
> +	if (n < 64) {
> +		res.hi = x.hi << n;
> +		res.hi |= x.lo >> (64 - n);

This is undefined when n == 0.

> +		res.lo = x.lo << n;
> +	} else {
> +		res.lo = 0;
> +		res.hi = x.lo << (n - 64);
> +	}
> +
> +	return res;
> +}
> +#endif /* shl_u128 */
> +
> +#ifndef shr_u128
> +static inline u128 shr_u128(u128 x, unsigned int n)
> +{
> +	u128 res;
> +
> +	if (n < 64) {
> +		res.lo = x.lo >> n;
> +		res.lo |= x.hi << (64 - n);

Likewise.

> +		res.hi = x.hi >> n;
> +	} else {
> +		res.hi = 0;
> +		res.lo = x.hi >> (n - 64);
> +	}
> +
> +	return res;
> +}
> +#endif /* shr_u128 */

Jay.
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