Re: [avr-gcc-list] Urgent Queries about AVR-GCC

[David Gay]

Joerg Wunsch wrote:
> "Suresh B Joshi" <address@hidden> wrote:
> > (2) When using the switch statement, even if the control
> >    expression is an unsigned char variable, it is promoted
> >    to 16 bit unsigned int, by eor R25,R25 instruction.
>
>
> IMHO, this is required by the C standard.  Also, you quote enums as an
> example, they are (unfortunately) always equivalent to type "int".

IMNSHO that's a bogus argument. Sure, the C standard says that the 
standard integer promotions are applied to the switch expression. But a 
compiler is only ever required to behave "as-if" the standard is 
followed, so in the case of a switch on an unsigned 8-bit integer, it 
can discard all cases < 0 and > 255, then do byte comparisons.
Of course, changing gcc to do this is probably not trivial...

An example where gcc does do this kind of optimisation is:

  char a, b;
  ...
  if (a == b) { ... }

The C rules say that a and b are promoted to int before the comparison, 
but gcc (of course) chooses to compare them with a byte comparison.

An example that generates large code which would be hard to remove is:

  signed char x = ...;
  unsigned char y = ...;

  if (x == y) { ...
  }

The comparison has to promote both x and y to int, then compare them 
(the compiler could also choose to test the high bit of x or y and fail 
the comparison if it fails, but that shouldn't be much shorter).

David Gay
address@hidden