Package: g++-4.9
Version: 4.9.2-10
Using g++ 4.9.2-10 on amd64, the below program behaves differently
when compiled with -O2 vs. not. The non-optimized version is correct;
the program's output should be:
~A(): moved = true
~A(): moved = false
However, with the optimized version, the program incorrectly outputs:
~A(): moved = false
~A(): moved = false
I.e. the line `other.moved = true` has been elided, or perhaps written
to the wrong location in memory. In practice, given the typical use
cases for move constructors, this bug tends to cause double-frees and
use-after-frees. In praticular, this is breaking the Cap'n Proto
library, which I maintain.
The attached self-contained script will demonstrate the problem.
Both G++ 4.8.4-1 and Clang 3.5.0-10 produce the correct output.
Additionally, G++ "4.9.2 20150304 (prerelease)" on Arch Linux produces
the correct output, possibly suggesting that this bug is
Debian-specific. (You can test different compiler versions by passing
the compiler command to use as the script's first parameter. By
default it uses `g++`.)
-Kenton
=======Program text=======
#include <stdio.h>
class A {
public:
bool moved = false;
A() = default;
A(A&& other) {
other.moved = true;
}
~A() {
printf("~A(): moved = %s\n", moved ? "true" : "false");
}
};
class B: virtual public A {};
class C: virtual public B {};
class D: public C {};
int main() {
A a = D();
}
=========================
-- System Information:
Debian Release: 8.0
APT prefers testing-updates
APT policy: (500, 'testing-updates'), (500, 'testing')
Architecture: amd64 (x86_64)
Foreign Architectures: i386
Kernel: Linux 3.16.0-4-amd64 (SMP w/12 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
Versions of packages g++-4.9 depends on:
ii gcc-4.9 4.9.2-10
ii gcc-4.9-base 4.9.2-10
ii libc6 2.19-15
ii libcloog-isl4 0.18.2-1+b2
ii libgmp10 2:6.0.0+dfsg-6
ii libisl10 0.12.2-2
ii libmpc3 1.0.2-1
ii libmpfr4 3.1.2-2
ii libstdc++-4.9-dev 4.9.2-10
ii zlib1g 1:1.2.8.dfsg-2+b1
g++-4.9 recommends no packages.
Versions of packages g++-4.9 suggests:
ii g++-4.9-multilib 4.9.2-10
pn gcc-4.9-doc <none>
pn libstdc++6-4.9-dbg <none>
-- no debconf information
Attachment:
test-move-from-virtual-subclass.sh
Description: Bourne shell script