AVR-Chat

In AVR-Chat@yahoogroups.com, Don Kinzer wrote:

> For most languages, C included, it is conceptually
> easy [to determine whether or not a function modifies
> a global variable]: look at a function and see what
> global variables it might modify and then do the same
> for any function that it might call and so on.

But, doesn't a C compiler, by the very design of the language, limit itself to the consideration of a single translation unit at a time?  That would mean that if a called function is in a different unit there is no way of knowing what global variables are touched.  And, if the variable *isn't* modified by a function in a different unit, why would it be "global" in the first place?

> ... there are situations that complicate matters (e.g.
> writing through a pointer having an unknown value).  

So, even if the function is in the same unit, if it writes through a pointer the compiler again has to throw up its hands.  (This applies to file-scope variables, too.)

> If you compile the example code below ...

I wonder if the inlining is done first, in which case the optimization is unremarkable.

Graham Davies
ECROS Technology
www.ecrostech.com