Re: [Minios-devel] [UNIKRAFT PATCHv4 16/43] uk/arch: Implement ukarch_find_lsbit for Arm64

[Sharan Santhanam <sharan.santhanam@xxxxxxxxx>]

Hello Wei Chen,

Please find my comment(s) inline.

On 07/06/2018 11:03 AM, Wei Chen wrote:
> This function is ported from Arm32 and is modified to work
> on Arm64.
>
> Signed-off-by: Wei Chen <Wei.Chen@xxxxxxx>
> ---
>   include/uk/arch/arm/arm64/atomic.h | 64 ++++++++++++++++++++++++++++++
>   1 file changed, 64 insertions(+)
>   create mode 100644 include/uk/arch/arm/arm64/atomic.h
>
> diff --git a/include/uk/arch/arm/arm64/atomic.h 
> b/include/uk/arch/arm/arm64/atomic.h
> new file mode 100644
> index 0000000..53f010b
> --- /dev/null
> +++ b/include/uk/arch/arm/arm64/atomic.h
> @@ -0,0 +1,64 @@
> +/* SPDX-License-Identifier: BSD-2-Clause */
> +/*
> + * Port from Mini-OS: include/arm/os.h
> + */
> +/*
> + * Copyright (c) 2009 Citrix Systems, Inc. All rights reserved.
> + *
> + * Redistribution and use in source and binary forms, with or without
> + * modification, are permitted provided that the following conditions
> + * are met:
> + * 1. Redistributions of source code must retain the above copyright
> + *    notice, this list of conditions and the following disclaimer.
> + * 2. Redistributions in binary form must reproduce the above copyright
> + *    notice, this list of conditions and the following disclaimer in the
> + *    documentation and/or other materials provided with the distribution.
> + *
> + * THIS SOFTWARE IS PROVIDED BY AUTHOR AND CONTRIBUTORS ``AS IS'' AND
> + * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
> + * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
> + * ARE DISCLAIMED.  IN NO EVENT SHALL AUTHOR OR CONTRIBUTORS BE LIABLE
> + * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
> + * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
> + * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
> + * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
> + * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
> + * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
> + * SUCH DAMAGE.
> + */
> +
> +#ifndef __UKARCH_ATOMIC_H__
> +#error Do not include this header directly
> +#endif
> +
For finding the lsbit shouldn't we use the builtin function 
__builtin_ffs, __builtin_ffsl?
> +/**
> + * ukarch_find_lsbit - find first (lowest) set bit in word.
> + * @word: The word to search
> + *
> + * Undefined if no bit exists, so code should check against 0 first.
> + */
> +static inline unsigned long ukarch_find_lsbit(unsigned long word)
> +{
> +       int clz;
> +
> +       /* xxxxx10000 = word
> +        * xxxxx01111 = word - 1
> +        * 0000011111 = word ^ (word - 1)
> +        *      4     = 63 - clz(word ^ (word - 1))
> +        */
> +
> +       __asm__("sub x0, %[word], #1\n"
> +               "eor x0, x0, %[word]\n"
> +               "clz %[clz], x0\n"
> +               :
> +               /* Outputs: */
> +               [clz] "=r"(clz)
> +               :
> +               /* Inputs: */
> +               [word] "r"(word)
> +               :
> +               /* Clobbers: */
> +               "x0");
> +
> +       return 63 - clz;
> +}
Thanks & Regards
Sharan

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