Re: Optimizing structure layout

erik.schnetter@student.uni-tuebingen.de writes:
  > While it is quite common that compilers optimize the code they
  > produce, I haven't heard of a commonly used system that really
  > optimizes the layout of the data structures that are generated. Are
  > there such systems?


Michael Meissner wrote:
> For many languages, it is impossible. C for one requires that
> structure members be laid out in the order the programmer wrote
> them. I believe C++ allows classes to be optimized, but again, not
> structures.


No, class and struct aren't different at all.


The only case where th order is unspecified is like this one:


struct T { // or class
          int i;
public:
          int j;
};


The position of i compared to j is unspecified; this
isn't sufficient to do usefull optimisations.


IMO this is bad; it prevent optimisations such as:


struct T {
          char c1;
          int i;
          char c2;
};


sizeof (T) == 12


vs


struct T2 {
          int i;
          char c1;
          char c2;
};


sizeof (T2) == 8


Layout of T1
pos size type name
0 1 char c1
1 3 padding
4 4 int i
8 1 char c2
9 3 padding
12


Layout of T2
pos size type name
0 4 int i
4 1 char c1
5 1 char c2
6 3 padding
8
--


Valentin Bonnard
mailto:bonnardv@pratique.fr
http://www.pratique.fr/~bonnardv (Informations sur le C++ en Francais)
--

Return to the comp.compilers page.
Search the comp.compilers archives again.