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Hello, I was browsing through older posts that deal with the painful issue of portability (http://www.keil.com/forum/docs/thread8109.asp). I was (and still am) a big advocate of programming as much as possible conforming to the C standard, and having a layered structure that allowed "plugging-in" other hardware. But I have come to change my mind recently. I am reading the "ARM system developer's guide" (excellent book by the way. I'm reading it because I want to port some C167 code to an ARM9 environment) in which chapter 5 discusses writing efficient C code for an ARM. The point is, and it is fairly demonstrated, that even common, innocent looking C code can either be efficient of very inefficient on an ARM depending on specific choices made, let alone another processor used! So, if we are talking about squeezing every clock cycle out of a microcontroller - I do not believe that portability without ultimately littering the code is possible!
The key point is that the loop counter should count down to zero rather than counting up to some arbitrary limit.
Given that the loop counter is not used in or after the body of the loop the compiler is, I believe, well within its rights under the 'as if' rule to rearrange the loop to decrement rather than increment. I guess it's a quality of implementation issue.
If you decide to code loops like this to decrement rather than increment the resulting 'C' is no less portable, so I'm not entirely sure what your point is.
If you mean that you would have to perform this kind of manual optimisation for each platform and/or compiler you target then I congratulate you on being able to design hardware that is only just powerful enough to work with optimal code every time.
If 'every clock cycle counts' then you have to use assembly. 'C' will always produce code that is slower and larger - the problem is that you cannot predict by exactly how much. If this matters, don't use 'C'.
Say, Jack, are you going to read the manual for a change
I don't use ARM.