Re: [avr-gcc-list] Byte values promted to int when during compare

[Ned Konz]

On Thursday 24 June 2004 4:06 am, Joerg Wunsch wrote:
> "Lars Jeppesen (address@hidden)" <address@hidden> wrote:
> > It seems that A is promoted to int before the binary & operation ?
> > But why?
>
> Because the C standard says so.

Though of course you could use the
        -mint8
GCC option which makes ints 8 bits wide, and then be explicit about your data 
sizes (chars and ints will be 1 byte, longs 2 bytes, "long long" 4 bytes).


I've noticed that integer promotion rules are one of the first things that 
vendors of 8-bit microcontroller C compilers tend to change (or at least 
provide an option for).

For instance, the Microchip C18 compiler Users Guide says (section 2.7):

------

2.7 ISO DIVERGENCES

2.7.1 Integer Promotions

ISO mandates that all arithmetic be performed at int precision or greater. By 
default, MPLAB C18 will perform arithmetic at the size of the largest 
operand, even if both operands are smaller than an int. The ISO mandated 
behavior can be instated via the -Oi command-line option.

For example:

unsigned char a, b;
unsigned i;

a = b = 0x80;
i = a + b;      /* ISO requires that i == 0x100, but in C18 i == 0 */

Note that this divergence also applies to constant literals. The chosen type 
for constant literals is the first one from the appropriate group that can 
represent the value of the constant without overflow.

For example:

#define A 0x10 /* A will be considered a char unless -Oi specified */
#define B 0x10 /* B will be considered a char unless -Oi specified */
#define C (A) * (B) unsigned i; i = C; /* ISO requires that i == 0x100, but in 
C18 i == 0 */

------

Their other ISO divergences include:

* rom, ram, near, and far storage qualifiers (some ports of GCC have the "far" 
and "near" function attributes (as well as "model", etc.)

* overlay storage class (for local variables)

* static function arguments

* 0b0000001  style binary literals

* Automatically placing string literals in program memory (that is, "const rom 
char []")

Their language extensions include:

* anonymous structures inside unions (like GCC)

* inline assembly language delimited by _asm and _endasm 

* #pragmas for specifying memory sections and placing data or code in them 
(with optional absolute addresses) (like GCC's 
section("section-name") attribute, or its "shared" and "common" attributes)

* #pragmas for giving the compiler a hint as to where variables will be 
located at link time

* #pragmas to declare functions to be interrupt service routines (like 
GCC-AVR's "signal" function attribute)

* "short long" type for 24-bit numbers (somewhat like GCC's "long long" type)


Of course, GCC is notable for its extensions to the C language as well (see 
http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html). From the point of view 
of an AVR (and maybe also MSP430 and HC12) microcontroller user, I'd rather 
see some of the above extensions than some of the ones that are already in 
GCC.


-- 
Ned Konz
http://bike-nomad.com
GPG key ID: BEEA7EFE