John Porter <[EMAIL PROTECTED]> writes:
> Russ Allbery wrote:
>> It looks like I was misremembering; I remember a proposal for a "pure"
>> attribute in gcc, but it looks like the attribute used for functions
>> with no memory references and no side effects is "const" (a la C++). I
>> think "pure" was proposed for the somewhat relaxed version of that that
>> allowed memory references but not side effects.
> Are you sure? That sounds totally backwards to me. Declaring a
> function "const" is a promise that it's not going to change anything
> outside its call frame. But a pure function, in the math sense, doesn't
> even look at anything outside its call frame.
I'm sure about the definition of const:
`const'
Many functions do not examine any values except their arguments,
and have no effects except the return value. Such a function can
be subject to common subexpression elimination and loop
optimization just as an arithmetic operator would be. These
functions should be declared with the attribute `const'. For
example,
int square (int) __attribute__ ((const));
says that the hypothetical function `square' is safe to call fewer
times than the program says.
The attribute `const' is not implemented in GNU C versions earlier
than 2.5. An alternative way to declare that a function has no
side effects, which works in the current version and in some older
versions, is as follows:
typedef int intfn ();
extern const intfn square;
This approach does not work in GNU C++ from 2.6.0 on, since the
language specifies that the `const' must be attached to the return
value.
Note that a function that has pointer arguments and examines the
data pointed to must *not* be declared `const'. Likewise, a
function that calls a non-`const' function usually must not be
`const'. It does not make sense for a `const' function to return
`void'.
My memory of "pure" is from mailing list traffic that I don't have on
hand.
--
Russ Allbery ([EMAIL PROTECTED]) <http://www.eyrie.org/~eagle/>
