I just learned of P2815, and I do not know of another place I can respond to it, so I'm going to answer here.
The presentation's title slide (and unfortunately nowhere else — I missed it the first time too!) refers to P2188:
which, being a WG21 paper, contains contact info for the author. I've cc'ed him here.
In my opinion, the proposal in question eliminates all provenance based alias analysis, used to eliminate memory accesses when unanalyzed code is present.
Consider the following example code:
int foo(int* p){
int i;
opaque_operation(&i,p);
i = 0;
*p = 1;
return i;
}
(Where the optimizer is unable to analyze `opaque_operation`
In current compilers, this can be optimized to [...
int foo(int *p) {
int i; opaque_operation(&i, p);
*p = 1; return 0;
}
...]. Under the proposal, however, `opaque_operation` could observe that `&i` and `p` have the same *value representation*, which means that they must be equal (it may do so with defined behaviour in all executions, for example, by terminating if they are bytewise unequal). Thus the write to `p` may modify the value of `i`. This would make the transformation [...] invalid under the proposal.
I see your point. It would be like: You go to a magic show. The magician gives one deck of cards to an audience member, who randomly selects a card and signs its face. He gives a second deck of cards to a second audience member, who randomly selects another card and signs its face. The magician takes both cards behind a partition, then re-emerges and triumphantly shows the audience that both audience members have somehow chosen the same card!
"I know how that trick is done," you whisper. "Whenever the two audience members choose different cards, the magician goes behind the partition and kills himself."
As in magic, C++ has some common-sense rules about what counts as a reasonable explanation for a trick and what doesn't.
For example, all three major compilers will optimize
int bar(int k) {
while (k) { }
return 42;
}
to
int bar(int k) {
return 42;
}
because C++ considers the explanation "Whenever k==0, the bottom of the loop is never reached" to be unreasonable.
Now, personally, I actually disagree with C++'s criterion there — I think "sometimes the loop is infinite" seems like a pretty reasonable "explanation of the trick" as far as I'm concerned. But the example does show that C++ already has mechanisms by which it can say "Such-and-such an explanation of the trick is unreasonable; we don't have to entertain the possibility that it could ever really happen that way."
So I don't think you're correct to worry that P2188 / P2815 would completely destroy all provenance-based optimizations. It needs to propose some wording before we can say anything concrete about it, of course. And it might end up destroying some optimizations. But it needn't destroy all of them. And at first glance it doesn't seem out-of-keeping with the direction of C++.
(Likewise, the existence of `reinterpret_cast` doesn't destroy all type-based alias analysis. And `const_cast` famously destroys some-but-not-all const-based optimizations.)
–Arthur