mudflap cache question

mudflap cache question

[Herman ten Brugge]

I just read the mudflap pdf file and wanted to know how the cache was 
implemented.
I looked at the code in mf-runtime.c and found the cache functions. The 
code for the
__mf_uncache_object looks strange to me however.
The code looks like:

static void
__mf_uncache_object (__mf_object_t *old_obj)
{
  /* Remove any low/high pointers for this object from the lookup cache.  */

  /* Can it possibly exist in the cache?  */
  if (LIKELY (old_obj->read_count + old_obj->write_count))
    {
      uintptr_t low = old_obj->low;
      uintptr_t high = old_obj->high;
      unsigned idx_low = __MF_CACHE_INDEX (low);
      unsigned idx_high = __MF_CACHE_INDEX (high);
      unsigned i;
      for (i = idx_low; i <= idx_high; i++)
        {
          struct __mf_cache *entry = & __mf_lookup_cache [i];
          /* NB: the "||" in the following test permits this code to
             tolerate the situation introduced by __mf_check over
             contiguous objects, where a cache entry spans several
             objects.  */
          if (entry->low == low || entry->high == high)
            {
              entry->low = MAXPTR;
              entry->high = MINPTR;
            }
        }
    }
}

__MF_CACHE_INDEX(ptr) is (ptr >> 2) & 0x3ff at startup.

Suppose we have an old_obj->low of 0x10000 and an old_obj->high of 
0x20000. Then the
cache entries we look at is just entry 0. Which I believe is just wrong. 
Any pointer
between 0x10000 and 0x20000 must fill other cache entries. It gets even 
worse when
the old_obj->low = 0x1fff0 and object high is 0x20000. Then we probably 
won't
invalidate any cache entry.
Should I write a bug report?

     Herman.

Re: mudflap cache question

[Frank Ch. Eigler]

Hi -


Herman ten Brugge <hermantenbrugge@home.nl> writes:

> [...]
> I looked at the code in mf-runtime.c and found the cache
> functions. The code [...]
> __mf_uncache_object (__mf_object_t *old_obj)
>  [...]
>       unsigned idx_low = __MF_CACHE_INDEX (low);
>       unsigned idx_high = __MF_CACHE_INDEX (high);
>       for (i = idx_low; i <= idx_high; i++)
>           [... check cache slot at index i ...]
> [...]
> __MF_CACHE_INDEX(ptr) is (ptr >> 2) & 0x3ff at startup.
>
> Suppose we have an old_obj->low of 0x10000 and an old_obj->high of
> 0x20000. Then the cache entries we look at is just entry 0. Which I
> believe is just wrong.  Any pointer between 0x10000 and 0x20000 must
> fill other cache entries. [...]

You're right, one can't treat this way a data structure that is trying
to be a hash table.  Probably this wasn't found before because it
would result in false no-error indications, which are quiet.

> Should I write a bug report?

Not necessary, this is fine.  Thank you.


- FChE