Smart human for you, someone obsessed with micro-optimization but lacking knowledge of compiler optimizations for me..

Writing it for human readers can be used to argue for the original implementation if you think of for(i=0;i<expr;i++) as the C idiom for "iterate expr times" and here you want expr to equal the length of the string, which is what the (obviously pure) function yields. No unnecessary variables and assignments -- no clutter. It perfectly describes the intent.

This conversation started when another developer was alarmed that the strlen() call in the for loop looked like it could be quadratic. I think we owe that dev the respect to not dismiss their concern as being "obsessed with micro-optimization but lacking knowledge of compiler optimizations".

I try to write code that is as plain and simple as possible, and make it obvious what it does and that it has no gotchas. I want my code to be understandable both for new developers, and for my future self, who will surely be less smart than I think I am today.

Here is the original code, with the loop body elided:

  for( int i = 0;  i < strlen(s);  i++ ) {
  }

It is trivial to rewrite this as:

  for( int i = 0, n = strlen(s);  i < n;  i++ ) {
  }

This is a very common idiom, and now it is perfectly clear what the code actually does. Obviously, it only calls strlen() once.

Of course, as I pointed out in another comment, if you're writing a loop that iterates over a C string, you never have to call strlen() at all! You can just use the canonical C string loop:

  for( int i = 0;  s[i] != '\0';  i++ ) {
  }

Or if you like brevity (which I like too):

  for( int i = 0;  s[i];  i++ ) {
  }