RE: [avr-gcc-list] Inconsisten behaviour of switch statement

[Dave Hansen]

From: Eric Weddington <address@hidden>

> > From: Schwichtenberg, Knut [mailto:address@hidden
[...]
> > Eric, the point is not that I don't like the output, but the
> > case to be
> > selected should be deterministic. I always thought that a switch
> > statement would lead to identical results if the input value is
> > identical. This is currently not true. The table jump for a volatile
> > variable is identical to an if-then-else structure of a non-volatile
> > variable and the current if-then-else implementation for a volatile
> > variable can not be simulated by a table jump.
>
> This is one of many reasons why I personally don't like switch statements 
> in
> embedded code. I still suggest coding your algorithm as a dispatch table.
> Give it a try.

I think Knut has a point.  At least, if I understand what's going on.  Given 
code like

  switch (var)
  {
     <etc.>
  }

If var is volatile, it should be read only once, regarless of the number of 
cases.  Consider code like

  switch (fn())
  {
     <etc.>
  }

How many times would you expect fn to be called?

Of course, a simple workaround would be to read the value of var into a 
non-volatile local variable, and switch on that, e.g.,

  local = var;
  switch (local)
  {
     <etc.>
  }

But he shouldn't _have_ to do that.  Bug in gcc?

Regards,
  -=Dave

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