Re: [Qemu-devel] [PATCH v3 1/2] bitmap: get last word mask from nr directly

From: Paolo Bonzini <pbonzini@redhat.com>
To: Wei Yang <richardw.yang@linux.intel.com>, qemu-devel@nongnu.org
Cc: corentincj@iksaif.net, pl@kamp.de, kraxel@redhat.com,
	peterx@redhat.com, quintela@redhat.com
Subject: Re: [Qemu-devel] [PATCH v3 1/2] bitmap: get last word mask from nr directly
Date: Mon, 12 Aug 2019 18:18:43 +0200	[thread overview]
Message-ID: <9d456eac-5d7f-570a-b73e-a55f2068579f@redhat.com> (raw)
In-Reply-To: <20190718010456.4234-2-richardw.yang@linux.intel.com>

On 18/07/19 03:04, Wei Yang wrote:
> The value left in nr is the number of bits for the last word, which
> could be calculate the last word mask directly.
> 
> Remove the unnecessary size.

Hi,

the value left in nr is _not_ the number of bits for the last word if
the start and the end are in the same word.  For example, if start %
BITS_PER_LONG was 3 and nr == 1, you'd have:

- before the patch BITMAP_LAST_WORD_MASK(4)

- after the patch BITMAP_LAST_WORD_MASK(1)

Paolo

> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> 
> ---
> v2: refine bitmap_set_atomic too, suggested from Peter
> ---
>  util/bitmap.c | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/util/bitmap.c b/util/bitmap.c
> index 1753ff7f5b..5b15249796 100644
> --- a/util/bitmap.c
> +++ b/util/bitmap.c
> @@ -160,7 +160,6 @@ int slow_bitmap_andnot(unsigned long *dst, const unsigned long *bitmap1,
>  void bitmap_set(unsigned long *map, long start, long nr)
>  {
>      unsigned long *p = map + BIT_WORD(start);
> -    const long size = start + nr;
>      int bits_to_set = BITS_PER_LONG - (start % BITS_PER_LONG);
>      unsigned long mask_to_set = BITMAP_FIRST_WORD_MASK(start);
>  
> @@ -174,7 +173,7 @@ void bitmap_set(unsigned long *map, long start, long nr)
>          p++;
>      }
>      if (nr) {
> -        mask_to_set &= BITMAP_LAST_WORD_MASK(size);
> +        mask_to_set &= BITMAP_LAST_WORD_MASK(nr);
>          *p |= mask_to_set;
>      }
>  }
> @@ -182,7 +181,6 @@ void bitmap_set(unsigned long *map, long start, long nr)
>  void bitmap_set_atomic(unsigned long *map, long start, long nr)
>  {
>      unsigned long *p = map + BIT_WORD(start);
> -    const long size = start + nr;
>      int bits_to_set = BITS_PER_LONG - (start % BITS_PER_LONG);
>      unsigned long mask_to_set = BITMAP_FIRST_WORD_MASK(start);
>  
> @@ -208,7 +206,7 @@ void bitmap_set_atomic(unsigned long *map, long start, long nr)
>  
>      /* Last word */
>      if (nr) {
> -        mask_to_set &= BITMAP_LAST_WORD_MASK(size);
> +        mask_to_set &= BITMAP_LAST_WORD_MASK(nr);
>          atomic_or(p, mask_to_set);
>      } else {
>          /* If we avoided the full barrier in atomic_or(), issue a
> @@ -221,7 +219,6 @@ void bitmap_set_atomic(unsigned long *map, long start, long nr)
>  void bitmap_clear(unsigned long *map, long start, long nr)
>  {
>      unsigned long *p = map + BIT_WORD(start);
> -    const long size = start + nr;
>      int bits_to_clear = BITS_PER_LONG - (start % BITS_PER_LONG);
>      unsigned long mask_to_clear = BITMAP_FIRST_WORD_MASK(start);
>  
> @@ -235,7 +232,7 @@ void bitmap_clear(unsigned long *map, long start, long nr)
>          p++;
>      }
>      if (nr) {
> -        mask_to_clear &= BITMAP_LAST_WORD_MASK(size);
> +        mask_to_clear &= BITMAP_LAST_WORD_MASK(nr);
>          *p &= ~mask_to_clear;
>      }
>  }
> 