compiler optimizations

Hello,

1. What is this different style of value assigning? What is the use?

Unsigned char const event_strings[11] = {
~0x3f, ~0x06, ~0x5b, ~0x4f, ~0x66, ~0x6d, ~0x7d, ~0x07, ~0x7f, ~0x6f, ~0x00};

2. What are the drawbacks / overheads on the controller if we use volatile type? Can we assign volatile type to structure members and bdata variables as shown below:

Eg 1:

typedef struct dtmf_scan_block{
  unsigned char state;
  unsigned char user;
  unsigned char s_r;
  unsigned char r_id;
}DTMF_SCAN_BLOCK;

DTMF_SCAN_BLOCK volatile xdata dtmf[NO_DTMF];

Eg2:

unsigned char volatile bdata dtvar;
sbit dt0        = dtvar^0;
sbit dt1        = dtvar^1;
sbit dt2        = dtvar^2;
sbit dt3        = dtvar^3;

Eg3:

typedef struct extention_data
{
        unsigned char volatile dgt_buf[40];
        unsigned char volatile how[16];
        unsigned char call_privacy[3];
        unsigned char id;
}EXTN_DATA;

3. I am comparing bit type variable with unsigned char variable at many places in my code. Should I follow typecasting?

4. Is it necessary to turn off compiler optimizations {Level 9} for hardware driver initialization code in my project? I am initializing drivers for switch array MT8816, GM16C550 - UART, RTC - DS1380, DTMF IC MT8888,etc

please advise.