more questions about efficient C code

[shekita at crystal.UUCP]

I am currently modifying C code, written by someone else,
that is incredibly terse. It's paramount that the code
be fast, so I presume the code is terse for speed. I'm
now curious about a few things:

1) Is the "var = (bool expr) ? : expr1 : expr2" construct faster
   than just a good old if statement? 

2) Are for loops faster than while loops? 

3) Is there really much to be gained by using assignment within
   a boolean expression, for example, is

	if ((fp = fopen("foo", "r") == NULL) {
		 ...
	}

   really that much faster than

	fp = fopen("foo", "r");
	if (fp == NULL) {
		...
	}

 4) Are tests for zero more efficently written as

	if (var) {
		...
	}

    rather than as

	if (var != 0) {
		...
	}