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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