[Mesa-dev] [PATCH v2 07/12] mesa: glGet: simplify the 'enum not found' condition

[Kristian Høgsberg]

On Tue, Sep 11, 2012 at 11:47 AM, Brian Paul <brianp at vmware.com> wrote:
> On 09/10/2012 12:41 AM, Imre Deak wrote:
>>
>> When traversing the hash table looking up an enum that is invalid we
>> eventually reach the first element in the descriptor array. By looking
>> at the type of that element, which is always TYPE_API_MASK, we know that
>> we can stop the search and return error. Since this element is always
>> the first it's enough to check for its index being 0 without looking at
>> its type.
>>
>> Later in this patchset, when we generate the hash tables during build
>> time, this will allow us to remove the TYPE_API_MASK and related flags
>> completly.
>>
>> Signed-off-by: Imre Deak<imre.deak at intel.com>
>> ---
>>   src/mesa/main/get.c |    8 +++++---
>>   1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/src/mesa/main/get.c b/src/mesa/main/get.c
>> index 63fe296..48c6911 100644
>> --- a/src/mesa/main/get.c
>> +++ b/src/mesa/main/get.c
>> @@ -2012,16 +2012,18 @@ find_value(const char *func, GLenum pname, void
>> **p, union value *v)
>>      mask = Elements(table(api)) - 1;
>>      hash = (pname * prime_factor);
>>      while (1) {
>> -      d =&values[table(api)[hash&  mask]];
>> +      int idx = table(api)[hash&  mask];
>>
>>
>>         /* If the enum isn't valid, the hash walk ends with index 0,
>> -       * which is the API mask entry at the beginning of values[]. */
>> -      if (unlikely(d->type == TYPE_API_MASK)) {
>> +       * pointing to the first entry of values[] which doesn't hold
>> +       * any valid enum. */
>> +      if (unlikely(!idx)) {
>
>
> Minor nit, but I think "idx != 0" would be nicer here.
>
>
>>            _mesa_error(ctx, GL_INVALID_ENUM, "%s(pname=%s)", func,
>>                  _mesa_lookup_enum_by_nr(pname));
>>            return&error_value;
>>         }
>>
>> +      d =&values[idx];
>>
>>         if (likely(d->pname == pname))
>>            break;
>>
>
> To be honest, I'm not quite sure I understand how this code works and how we
> wind up at entry[0] for an invalid enum.

We use a hash table to map enum values to the index into the
corresponding value_desc table.  The first entry in value_desc table
is not a valid enum description, so we use index 0 as a terminator for
the hash chains.  So if we end up looking at value_desc[0], it means
that we reached the end of a hash chain and didn't file the value we
were looking for.

Kristian