On Thursday, 28 October 2021 at 14:49:41 UTC, Paul Backus wrote:

Some other change has been made to the spec after that PR was made. It currently reads:

void bad()
{
    int x = void;
    writeln(x);  // print implementation defined value
}

void muchWorse()
{
    char[] p = void;
    writeln(p);  // may result in apocalypse
}

On Thursday, 28 October 2021 at 15:18:54 UTC, Dukc wrote:

That's exactly what the linked issue (22277) is about, so if you have any more insights on that, please post them there.

On Thursday, 28 October 2021 at 15:40:21 UTC, Dennis wrote:

No that's slightly different. The linked issue deals with strongly pure functions. I'm dreaming about letting the compiler to optimise based on weak pure too - not currently allowed if I read the spec right, but could be without this issue (I think?).

On Thursday, 28 October 2021 at 15:18:54 UTC, Dukc wrote:

pure nothrow @nogc void free(int[])

Not at all. free cannot, by its semantic, be pure (same for malloc). Trying to make free pure is a silly challenge.

On Thursday, 28 October 2021 at 16:36:30 UTC, Dukc wrote:

Is this worth caring about? Do the backend a that actually matter not already perform this analysis as part of their IPA?

On Thursday, 28 October 2021 at 19:46:32 UTC, Patrick Schluter wrote:

https://dlang.org/phobos/core_memory.html#.pureFree

On Thursday, 28 October 2021 at 21:48:09 UTC, max haughton wrote:

You decide. Without that, the weak pure can still be used inside strongly pure functions, but is otherwise useless for optimisation.

If the function body is available and not too complicated, probably. But with pure it's possible for a compiler to optimise based on the signature alone.

I don't personally care that much about having a super-optimising compiler, but I still wish that our attributes provide as much info as possible for any analysis program.

On Thursday, 28 October 2021 at 23:05:04 UTC, Dukc wrote:

I still think this analysis is going to be done anyway by either GCC or LLVM. Also, is elision even valid in the general case, what if the function is weak. It can't be elided until link-time in some rare but not impossible circumstances.

On Thursday, 28 October 2021 at 22:48:10 UTC, Dukc wrote:

I don't understand it. It does not make any sense. pure functions are function that do not depend on global state for their result. Weak purity allows for some exception like for a print function which has global effects but these effects have no feedback and can ne ignored. This is not the case with allocation/free, which are, by defintion, dependend on a global state (even if only thread local).
Each call to malloc, by definition must return another value and/or can return a same value with other parameter. The result does not depend on ANYTHING in its scope.
pure allocation/free is a recipe for disaster.
My understanding of purity at l east.