On Thu, 21 Dec 2023, David Malcolm via Gcc wrote:
On Wed, 2023-12-20 at 11:16 -0800, Eric Batchelor wrote:
Hello, I unintentionally stumbled upon some strange behaviour that
occurred due to a typo.
I reproduced the behaviour where an object (std::string in my case)
can
be passed to a function by reference, uninitialized, WITHOUT a
compiler
warning.
Changing the code to pass the object by value DOES emit the warning.
I don't think the compiled code is incorrect, it segfaults presumably
due to uninitialized members.
I understand there may seldom be a reason to use uninitialized
objects,
so "don't do that," but as I said this was unintentional and it seems
that it should have generated a warning, which have saved some
head-scratching.
Code to reproduce:
#include <string>
std::string f(std::string &s) {
s.append("x");
return s;
}
int main() {
std::string a = f(a);
}
Compile and run (no warning):
$ g++ -o uninit_obj uninit_obj.cpp -std=c++23 -Wall -Wpedantic -
Wextra
&& ./uninit_obj
Segmentation fault (core dumped)
No difference whether using -O0 (or 1 2 3)
As I understand it, -Wmaybe-uninitialized is purely intraprocedural
i.e. it works within each individual function, without considering the
interactions *between* functions.
If you compile
#include <string>
static std::string f(std::string &s) {
s.append("x");
return s;
}
void g() {
std::string a = f(a);
}
with -O3, by the time we get to the uninit pass, function g starts with
void g ()
{
size_type __dnew;
struct string a;
[...]
<bb 2> [local count: 1073741824]:
_26 = a._M_string_length;
if (_26 == 4611686018427387903)
which should not require any interprocedural logic.
--
Marc Glisse
