Re: Data Structure Reorganizing Optimizations

Andy Glew <glew@ichips.intel.com> wrote:
[...]
>I am aware of the K&R rule that structure elements be in increasing
>order of address. But, since the contiguity requirement has been
>dropped, and since different machines have different padding rules,
>what value does the address ordering rule have? Even network header
>code cannot be written portably using structures to access data
>structures placed in memory from DMA devices.
>
>I believe that C's rules about data structure organization are
>obsolete. Certainly, they are not in the ken of the usual programmer
>using C or C++. Why not permit compilers to do such "illegal" data
>structure reorganizations, bringing the performance gains Lebeck and
>Wood describes to the standard application?


Have you ever seen code like this:


struct type1 {
    int a;
    char *b;
    char *c;
};


struct type2 {
    int a;
    float b;
    char * c;
};


union foo {
    type1 t1;
    type2 t2;
};


Then code looks at "foo.t1.a" to decide if foo is of type t1 or t2?
I beleve this sort of thing is at least somewhat common. I also think
Xlib does it (with the event structure). This may also be legal without
the union, but I find that distasteful (of corse I'm not real happy with
the union either...).


>I suggest that this be considered by future X3J11 ANSI C standards (if
>any such revisions are forthcoming).


Remember to deal with union's.
--

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