mirror of
https://github.com/topjohnwu/ndk-busybox.git
synced 2024-11-28 14:10:34 +00:00
40 lines
1.5 KiB
Plaintext
40 lines
1.5 KiB
Plaintext
smalluint i = index_in_str_array(params, name) + 1;
|
|
if (i == 0)
|
|
return 0;
|
|
if (!(i == 4 || i == 5))
|
|
i |= 0x80;
|
|
|
|
return i;
|
|
|
|
I think that this optimization is wrong.
|
|
index_in_str_array returns int. At best, compiler will use it as-is.
|
|
At worst, compiler will try to make sure that it is properly cast
|
|
into a byte, which probably results in "n = n & 0xff" on many architectures.
|
|
|
|
You save nothing on space here because i is not stored on-stack,
|
|
gcc will keep it in register. And even if it *is* stored,
|
|
it is *stack* storage, which is cheap (unlike data/bss).
|
|
|
|
small[u]ints are useful _mostly_ for:
|
|
|
|
(a) flag variables
|
|
(a1) global flag variables - make data/bss smaller
|
|
(a2) local flag variables - "a = 5", "a |= 0x40" are smaller
|
|
for bytes than for full integers.
|
|
Example:
|
|
on i386, there is no widening constant store instruction
|
|
for some types of address modes, thus
|
|
movl $0x0,(%eax) is "c7 00 00 00 00 00"
|
|
movb $0x0,(%eax) is "c6 00 00"
|
|
(b) small integer structure members, when you have many such
|
|
structures allocated,
|
|
or when these are global objects of this structure type
|
|
|
|
small[u]ints are *NOT* useful for:
|
|
|
|
(a) function parameters and return values -
|
|
they are pushed on-stack or stored in registers, bytes here are *harder*
|
|
to deal with than ints
|
|
(b) "computational" variables - "a++", "a = b*3 + 7" may take more code to do
|
|
on bytes than on ints on some architectires.
|