source: branches/3.2/mindi-busybox/docs/smallint.txt @ 3232

Last change on this file since 3232 was 2725, checked in by Bruno Cornec, 9 years ago
  • Update mindi-busybox to 1.18.3 to avoid problems with the tar command which is now failing on recent versions with busybox 1.7.3
  • Property svn:eol-style set to native
File size: 1.5 KB
Line 
1        smalluint i = index_in_str_array(params, name) + 1;
2        if (i == 0)
3                return 0;
4        if (!(i == 4 || i == 5))
5                i |= 0x80;
6
7        return i;
8
9I think that this optimization is wrong.
10index_in_str_array returns int. At best, compiler will use it as-is.
11At worst, compiler will try to make sure that it is properly cast
12into a byte, which probably results in "n = n & 0xff" on many architectures.
13
14You save nothing on space here because i is not stored on-stack,
15gcc will keep it in register. And even if it *is* stored,
16it is *stack* storage, which is cheap (unlike data/bss).
17
18small[u]ints are useful _mostly_ for:
19
20(a) flag variables
21    (a1) global flag variables - make data/bss smaller
22    (a2) local flag variables - "a = 5", "a |= 0x40" are smaller
23         for bytes than for full integers.
24            Example:
25            on i386, there is no widening constant store instruction
26            for some types of address modes, thus
27            movl $0x0,(%eax) is "c7 00 00 00 00 00"
28            movb $0x0,(%eax) is "c6 00 00"
29(b) small integer structure members, when you have many such
30    structures allocated,
31    or when these are global objects of this structure type
32
33small[u]ints are *NOT* useful for:
34
35(a) function parameters and return values -
36    they are pushed on-stack or stored in registers, bytes here are *harder*
37    to deal with than ints
38(b) "computational" variables - "a++", "a = b*3 + 7" may take more code to do
39    on bytes than on ints on some architectires.
Note: See TracBrowser for help on using the repository browser.