How to modify GCC so to insert some code for debug?

[John]

Hi,

I use GCC to compile the code that runs in a very resouse-limited embed system,
so I can't use the usual debug tools, such as GDB. And there are thousands of
lagacy code already, in which there is no debug code, sth like
PRINT(__FILE__, __LINE__).

So I want to modify the GCC to meet the requirement below:
1. In each funtion entry and exit, print out some info, such as the
funtion name, time, etc.

2. Before each loop(for/while/until) and branch selection, print out
the condition info.
   For exmaple:
        for(i = ...) while(j < ...), if(i > ...), print out the value of i.

3. More advanced, when a variable is accessed, its value is printed.
Like the watch command in GDB.

Could some gurus give me some suggestions? Such as is there a project
aimed at this? How long/difficult
does it take if I make it from scratch?

Thanks,
John