https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100746
Bug ID: 100746
Summary: NRVO should not introduce aliasing
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: antoshkka at gmail dot com
Target Milestone: ---
Consider the example:
struct NrvoPassed {
NrvoPassed() = default;
NrvoPassed(const NrvoPassed&);
NrvoPassed(NrvoPassed&&);
int i = 0;
};
auto test(int* data) {
NrvoPassed x;
*data = 3;
if (x.i != 0) __builtin_abort();
return x;
}
Resulting assembly contains call to `abort`:
test(int*):
mov DWORD PTR [rdi], 0
mov DWORD PTR [rsi], 3
mov edx, DWORD PTR [rdi]
test edx, edx
jne .L3
mov rax, rdi
ret
test(int*) [clone .cold]:
.L3:
push rax
call abort
Optimizer thinks that the value of `x.i` is aliased by `data`, however `data`
is a local variable and it's address could not leak before the object is
constructed.
Some other compilers already have the proposed optimization:
https://godbolt.org/z/aqdveadnE
Adding `__restrict` to `data` fixes the codegen:
test2(int*):
mov DWORD PTR [rdi], 0
mov rax, rdi
mov DWORD PTR [rsi], 3
ret
Probably `__restrict` should be always added to the storage address passed for
NRVO.
