8f8ee534a
Denys Vlasenko
2010-10-24
Subject
adding docs/smallint.txt
Binary
No binary (build failed)
Architecture & Compiler Flags
—
Build Log
(log yok)
Commit Diff GitHub →
docs/smallint.txt+37 -0
| @@ -0,0 +1,37 @@ | |
| + | 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 casted |
| + | 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 it is *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. |