Open Quuxplusone opened 3 years ago
Quuxplusone
commented
3 years ago Not that it changes the result, but the godbolt example doesn't actually run the attributor since it is not turned on by default yet.
This does: https://godbolt.org/z/TxxvKx
Quuxplusone
commented
3 years ago It was confirmed in the LWG mailing list that for a standard library API `foo`
void foo(int&& a, int&& b) { }
the behavior of:
int a = 42;
foo(move(a), move(a));
is undefined because the rvalue references passed to "foo" alias and that's not
allowed according to [res.on.arguments]/1.3.
---
From a libc++ point-of-view, this probably also means that the standard library
needs to be careful of not accessing any static or global from a function that
could accept that value as an argument, e.g.,
int b = 42;
void foo(int&& a) {
b += a; // UB if "a" aliases "b"
}
foo(move(b)); // UB
and standard library users need to be careful as well:
struct F {
int* p;
void operator()(int b) {}
};
int test() {
int a = 42;
std::async(
F{&a},
std::move(a) // "a" aliases "F.p"
);
}
According to the wording and notes in [res.on.arguments]/1.3, standard library APIs taking rvalue references arguments can assume that these do not alias.
When looking at: https://godbolt.org/z/G8ejKb
include
int fails(int p) { int&& i = std::move(std::move(p)); return i; } int works(int __restrict p) { return p; }
the last call to
std::movein "fails" takes an rvalue reference, and its argument should benoalias, which LLVM attributor pass should be able to propagate up top.However, the generated IR is missing no-alias.
I don't think the standard guarantees that rvalue references are always no alias, but the standard library does guarantee that for all its APIs.
The simplest solution I can imagine would be to add a [[std_lib_api]] builtin attribute that adds
noaliasto all rvalue reference arguments of a function. We'd need to implement the argument, and add it to libc++.Richard, Eric, thoughts?
To me this wording looks extremely powerful, but from https://cplusplus.github.io/LWG/issue1204 I don't know if this is intended or not.