Subj : Performance impact of disaligned structures
To   : comp.programming
From : Gioele Barabucci
Date : Tue Oct 04 2005 02:48 pm

How much "worse" is this structure

struct {
  uint64 length;
  uint32 id;
  uint64 *next;     /* this is 32 bits aligned */
  uint64 data[256]; /* this starts on the 32 bits boundary */
}

compared to this other

struct {
  uint64 length;
  uint32 id;
  uint32 padding;
  uint64 *next;     /* now this is 64 bits aligned */
  uint64 data[256]; /* and this starts on the 64 bits boundary */
}

on 32 bits and 64 bits architectures?

The '*next' pointer is going to be used seldom; what worries me is the
data field.
The content of data[256] is going to be used as input for heavy CPU
computations, it should be loaded as fast as possible into the
MMX/Altivec registers.

How (and how much) will the different layout affect run time speed? My
targets are x86 and ppc32 on the 32 bits side and x86_64 and POWER
(ppc64) on the 64 bits side.


-- 
Gioele Barabucci <barabucc@cs.unibo.it>

.