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Re: [libvirt] [PATCH v2 2/9] New functions for virBitmap



On 09/06/2012 04:13 AM, Hu Tao wrote:
> In many places we store bitmap info in a chunk of data
> (pointed to by a char *), and have redundant codes to
> set/unset bits. This patch extends virBitmap, and convert
> those codes to use virBitmap in subsequent patches.
> ---

>  
>  struct _virBitmap {
> -    size_t size;
> -    unsigned long *map;
> +    size_t size;        /* size in bits */
> +    size_t size2;       /* size in LONGs */

The name 'size2' isn't very descriptive.  Maybe we should rename to
s/size/max_bit/ and s/size2/map_len/ for easier reading?

> +    unsigned long *map; /* bits are stored in little-endian format */

This comment...

> +/* Helper function. caller must ensure b < bitmap->size */
> +static bool virBitmapIsSet(virBitmapPtr bitmap, size_t b)
> +{
> +    return !!(bitmap->map[VIR_BITMAP_UNIT_OFFSET(b)] & VIR_BITMAP_BIT(b));

...and this code disagree.  This code is reading in machine-native
format, not little-endian.  And I much prefer operating in
machine-native format.  Which means when converting from char* to long,
you'll have to properly pass things through endian conversion, rather
than requiring the longs to be little-endian.

> +char *virBitmapFormat(virBitmapPtr bitmap)
> +{
> +    virBuffer buf = VIR_BUFFER_INITIALIZER;
> +    int first = -1;
> +    int start, cur;
> +    int ret;
> +    bool isset;
> +
> +    if (!bitmap)
> +        return NULL;
> +
> +    cur = 0;
> +    start = -1;
> +    while (cur < bitmap->size) {
> +        ret = virBitmapGetBit(bitmap, cur, &isset);

I'm wondering if we should optimize this by using things like ffsl() and
iterating a long at a time for longs that are 0 or -1, rather than
blindly processing one bit at a time.  Or even make this use the new
virBitmapNextSetBit, and have that function be optimized a bit more.

> +        if (ret != 0)
> +            goto error;
> +        else if (isset) {

Style.  Since the else branch used {}, the if branch must also use {}.

> +            if (start == -1)
> +                start = cur;
> +        } else if (start != -1) {
> +            if (!first)
> +                virBufferAddLit(&buf, ",");
> +            else
> +                first = 0;
> +            if (cur == start + 1)
> +                virBufferAsprintf(&buf, "%d", start);
> +            else
> +                virBufferAsprintf(&buf, "%d-%d", start, cur - 1);
> +            start = -1;
> +        }
> +        cur++;
> +    }
> +
> +    if (start != -1) {
> +        if (!first)
> +            virBufferAddLit(&buf, ",");
> +        if (cur == start + 1)
> +            virBufferAsprintf(&buf, "%d", start);
> +        else
> +            virBufferAsprintf(&buf, "%d-%d", start, cur - 1);
> +    }
> +
> +    if (virBufferError(&buf)) {
> +error:
> +        virBufferFreeAndReset(&buf);
> +        return NULL;
> +    }
> +
> +    return virBufferContentAndReset(&buf);

Ouch.  If the bitset is completely unset, then this returns NULL for
both errors and success.  You need to special-case a map that is
completely unset to return strdup("") instead.

> +
> +#ifdef __BIG_ENDIAN__
> +static unsigned long
> +virSwapEndian(unsigned long l)

Yuck.  __BIG_ENDIAN__ vs. __LITTLE_ENDIAN__ is not guaranteed to exist.
 And even if you can rely on it, there's bound to be better ways to
implement this instead of open-coding it ourselves (not to mention that
by avoiding the #ifdefs, we avoid introducing bugs in the big-endian
code that cannot be detected on little-endian machines).  (Hmm, too bad
gnulib doesn't guarantee be32toh and be64toh).

> +/**
> + * virBitmapAllocFromData:
> + * @data: the data
> + * @len: length of @data in bytes
> + *
> + * Allocate a bitmap from a chunk of data containing bits
> + * information
> + *
> + * Returns a pointer to the allocated bitmap or NULL if
> + * memory cannot be allocated.
> + */
> +virBitmapPtr virBitmapAllocFromData(void *data, int len)
> +{
> +    virBitmapPtr bitmap;
> +#ifdef __BIG_ENDIAN__
> +    int i;
> +#endif
> +
> +    bitmap = virBitmapAlloc(len * CHAR_BIT);
> +    if (!bitmap)
> +        return NULL;
> +
> +    memcpy(bitmap->map, data, len);

Instead of trying to memcpy() and then conditionally virSwapEndian(), I
would just do it the manual way of reading one byte at a time for both
endian types.  Fewer #ifdefs, less ugly code.

> +int virBitmapToData(virBitmapPtr bitmap, char **data, int *dataLen)
> +{
> +    int len;
> +#ifdef __BIG_ENDIAN__
> +    unsigned long *l;
> +#endif
> +
> +    len = bitmap->size2 * (VIR_BITMAP_BITS_PER_UNIT / CHAR_BIT);
> +
> +    if (VIR_ALLOC_N(*data, len) < 0)
> +        return -1;
> +
> +    memcpy(*data, bitmap->map, len);
> +    *dataLen = len;
> +
> +#ifdef __BIG_ENDIAN__
> +    l = (unsigned long *)*data;
> +    for (i = 0; i < bitmap->size2; i++, l++)
> +        *l = virSwapEndian(*l);
> +#endif

Again, I'm not a fan of these #ifdefs.

> +int virBitmapNextSetBit(virBitmapPtr bitmap, int pos)
> +{
> +    int nl;
> +    int nb;
> +    unsigned long bits;
> +
> +    if (pos < 0)
> +        pos = -1;
> +
> +    pos++;
> +
> +    if (pos >= bitmap->size)
> +        return -1;
> +
> +    nl = pos / VIR_BITMAP_BITS_PER_UNIT;
> +    nb = pos % VIR_BITMAP_BITS_PER_UNIT;
> +
> +    bits = bitmap->map[nl] & ~((1UL << nb) - 1);
> +
> +    while (bits == 0 && ++nl < bitmap->size2) {
> +        bits = bitmap->map[nl];
> +    }

Use ffsl() instead of iterating one bit at a time.

> +
> +    if (bits == 0)
> +        return -1;
> +
> +    return __builtin_ctzl(bits) + nl * VIR_BITMAP_BITS_PER_UNIT;

__builtin_ctzl() is not guaranteed to exist.  ffsl() should already give
you what you need.


> +++ b/tests/virbitmaptest.c
> @@ -0,0 +1,233 @@

> +#include <config.h>
> +
> +#include <time.h>
> +#include <sched.h>

What are you using <time.h> and <sched.h> for?

It's late for me, so I didn't closely read the entire patch, so much as
identified things that jumped out on first glance.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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