Re: Alignment of structures.

[Etienne LORRAIN <etienne_lorrain at yahoo dot fr>]

  I think I can answer this one:

> If I add in an extra long into a struct definition the structures are not
> packed together, rather they are put on 32 byte boundary.  This seems to 
> be a bug in the Intel alignment.  I tried to trim this down to a very
small > example.

  Not a bug but a feature.

  In the early days of microprocessor, the execution time of software
 was reduced if you align address of each half word variable on a 2 bytes
 boundary , and align address of each word variable on 4 bytes boundary.
 (not speaking of microprocessor which cannot handle un-aligned access).

  Now, processor are faster if you align big structures (>= 31 bytes)
 on 32 bytes boundary - so the compiler will align your structures
 sckw01 and sckw59 to start at an 32 byte boundary - that explain
 the "zero filling" you get when "struct nkw" go from 32 bytes to 36
 bytes. Your software should not depend on the fact that two contigous
 variables would be contigous in memory.

  If you need to, you can declare:
static struct {
    NKW sckw0;
    NKW sckw56;
    NKW sckw65;
    } sckw = { {...}, {...}, {...} };
  And if you cannot touch the other source:
#define sckw0 sckw.sckw0
#define sckw56 sckw.sckw56
#define sckw65 sckw.sckw65

  You can also have a look on the "packed" attribute of types,
 in gcc documentation. You can also increase the alignment,
 but unfortunately not deacrease it, by using the "align"
 attribute, on the type or on the variable:
typedef struct nkw {...} __attribute__ ((align(64)));
  or:
struct nkw sckw0 __attribute__ ((align(64)));

  If you really need the previous system, you can recompile
 GCC after completely removing in
gcc-2.95.*/gcc/config/i386/i386.h
 the "#define":
CONSTANT_ALIGNMENT
DATA_ALIGNMENT
  and maybe:
LOCAL_ALIGNMENT

  The only real bad feature here is that you cannot reduce
 alignment (which can be 32, 64, 128 or 256 bytes) when
 you do not really care of the speed but more on the size
 of the executable (-Os option) of for only one variable.

  If you look deep in the documentation, you will find
 the "packed" attribute for variables - but it does not work.

  Hope that helps,
  Etienne.
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