linux-kernel - Re: [PATCH] lib: Avoid redundant sizeof checking in __bitmap

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Date:   Tue, 14 Nov 2017 08:23:44 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Rakib Mullick <rakib.mullick@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Yury Norov <ynorov@...iumnetworks.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Subject: Re: [PATCH] lib: Avoid redundant sizeof checking in __bitmap_weight() calculation.

On 14 November 2017 at 07:57, Rakib Mullick <rakib.mullick@...il.com> wrote:
> Currently, during __bitmap_weight() calculation hweight_long() is used.
> Inside a hweight_long() a check has been made to figure out whether a
> hweight32() or hweight64() version to use.
>
> diff --git a/lib/bitmap.c b/lib/bitmap.c
> index d8f0c09..552096f 100644
> --- a/lib/bitmap.c
> +++ b/lib/bitmap.c
> @@ -241,10 +241,15 @@ EXPORT_SYMBOL(__bitmap_subset);
>  int __bitmap_weight(const unsigned long *bitmap, unsigned int bits)
>  {
>         unsigned int k, lim = bits/BITS_PER_LONG;
> -       int w = 0;
> -
> -       for (k = 0; k < lim; k++)
> -               w += hweight_long(bitmap[k]);
> +       int w = 0, is32 = sizeof(bitmap[0]) ? 1 : 0;
> +

hint: sizeof() very rarely evaluates to zero... So this is the same as
"is32 = 1". So the patch as-is is broken (and may explain the 1-byte
delta in vmlinux). But even if this condition is fixed, the patch
doesn't change anything, since the sizeof() evaluation is done at
compile-time, regardless of whether it happens inside the inlined
hweight_long or outside. So it is certainly not worth it to duplicate
the loop.

Rasmus