Thread: Silent cast to unsigned

I'm a bit confused about this behaviour (gcc 4.1.2, Linux): i have function that accepts "char*" argument. Function is basicly my version of str(case)cmp() and I'm trying to optimize it for my locale and catch up with str(case)cmp(). I don't want to compile with any optimization flags. I found out that any extra argument given to function slows it down (1 more argument, and there's ~20% slowdown - tested on ~15000 strings comparing with each other). Another thing that causes slowdown is any excplicit cast (like signed to unsigned). As I use tables for fast conversion from uppercase to lowercase when that version is called (case insensitive), i just type: "c=TABLE[c]" (c is current character). Example:

Code:

int _f(char *s1, char *s2, int cs)
{
    // cs = case sensitivity (0=off)

    (register?) int i=0;      // index in string(s)
    (register?) int c1, c2;   // current char in 1st and 2nd strings

    for(;;)
    {
         c1 = s1[i];
         ...
         if (!cs) c1=TABLE[c1];
         ...
    }
}

(I'll ignore slowdown by "cs" argument for now...)

The thing that bugs me is that c1 CAN become negative (i've checked) and it's EXPECTED as no cast is being done, but then again when doing TABLE[c1] it gets silently cast to unsigned (as no crash occurs) when used as index. Is this normal or am i missing something?

EDIT: Oh yes, TABLE is plain "int TABLE[256]" initialized before _f() call...