On Thu, Sep 06, 2012 at 11:17:58PM -0600, Eric Blake wrote:
> 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?
Thanks for suggestion.
>
> > + 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.
OK.
>
> > +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.
Will improve it.
>
> > + if (ret != 0)
> > + goto error;
> > + else if (isset) {
>
> Style. Since the else branch used {}, the if branch must also use {}.
OK.
>
> > + 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.
OK.
>
> > +
> > +#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.
OK.
>
> > +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?
Oops. This is a copy&paste.
>
> 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.
Have a good night:)
--
Thanks,
Hu Tao
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