Re: [HACKERS] sbufdesc' padding...

> I'm going to remove subj...
>
> buf_internals.h:
>
> /*
> * I padded this structure to a power of 2 (PADDED_SBUFDESC_SIZE)
> * because BufferDescriptorGetBuffer is called a billion times and it
> * does an C pointer subtraction (i.e., "x - y" -> array index of x
> * relative to y, which is calculated using division by struct size).
> ^^^^^^^^^^^^^^^^^^^^^^^^
> * Integer ".div" hits you for 35 cycles, as opposed to a 1-cycle
> * "sra" ... this hack cut 10% off of the time to create the Wisconsin
> * database! It eats up more shared memory, of course, but we're
> * (allegedly) going to make some of these types bigger soon anyway...
> * -pma 1/2/93
> */
>
> This is not true now:
>
> #define BufferDescriptorGetBuffer(bdesc) ((bdesc)->buf_id + 1)
>
> Comments ?...
>
> Vadim
>
>

You can remove the comment about size-of-2. It currently pads to a
fixed size of 128, because the old code didn't work. We now do:

/*
* please, don't take the sizeof() this member and use it for
* something important
*/

char sb_relname[NAMEDATALEN + /* name of reln */
PADDED_SBUFDESC_SIZE - sizeof(struct sbufdesc_unpadded)];

which is much stranger, but works 100% of the time.

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