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From: "remi.galanalfonso at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/107200] False positive -Wdangling-pointer?
Date: Mon, 10 Oct 2022 19:38:44 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107200

R=C3=A9mi Galan Alfonso <remi.galanalfonso at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |remi.galanalfonso at gmail=
 dot com

--- Comment #3 from R=C3=A9mi Galan Alfonso <remi.galanalfonso at gmail dot=
 com> ---
Hello,

If I remember correctly, Eigen uses expression templates (and the names in =
the
inline stack make it look like it).
In that case your auto would not be Eigen::Vector2d (which you can see in y=
our
godbolt by adding e.g. "static_assert(std::is_same<decltype(x),
Eigen::Vector2d>::value, "Not Eigen::Vector2d.");"), it would be a type that
represents the expression "0.5 * Eigen::Vector2d{1.0, 2.0}", which probably
contains a reference to the temporary Vector2d, which is destroyed at the e=
nd
of the line, before the "x(0)" and "x(1)". And that would explain why you d=
on't
see the problem when you replace the auto with "Eigen::Vector2d", as in that
case that causes the evaluation of the expression template while the tempor=
ary
still exists.

Moreover, if you enable "Execute the code" on godbolt and add the option
"-fsanitize=3Daddress", it trigger the address sanitizer with auto and not
Vector2d, with "ERROR: AddressSanitizer: stack-use-after-scope".
(And bonus: when running the code without sanitizer in O3, what is printed =
is
not the "expected" result, for example I get "0, 1.03725e-317" on godbolt, =
GCC
optimized assuming the code doesn't use the dangling values)

So I think the warning is correct, but you probably want to wait for
confirmation from a GCC developer ;).=