For the last few days, I have been debugging a weird issue involving lambdas in C++. I have reduced the problem down to the following symptoms:

The this pointer gets corrupted inside a lambda (note: this is always captured by copy, so the lambda should get its own this pointer, which points to the App object)
It only occurs if a std::cout print statement is present, and called before the lambda is created. The print statement can be seemingly completely unrelated (e.g. print "Hello!"). printf() also exhibits the same behaviour.
It only occurs when cross-compiling.
It compiles and runs fine with the standard compiler for x86 architecture (see example).
If I create the lambda on the heap (and save a pointer to it inside the App object), the bug does not occur.
The bug does not occur if optimizations are turned off (i.e. if I set the -O0 flag). It occurs when optimization is set to -O2.

The following is the simplest, compilable code example I could come up with that causes the problem.

#include <iostream>
#include <functional>

class App {

public:

    std::function<void*()> test_;

    void Run() {

        // Enable this line, ERROR is printed
        // Disable this line, app runs o.k.
        std::cout << "This print statement causes the bug below!" << std::endl;

        test_ = [this] () {
            return this;
        };

        void* returnedThis = test_();
        if(returnedThis != this) {
            std::cout << "ERROR: 'this' returned from lambda (" << returnedThis 
                      << ") is NOT the same as 'this' (" << this << ") !?!?!?!?!"
                      << std::endl;
        } else {
            std::cout << "Program run successfully." << std::endl;
        }

    }
};

int main(void) {
    App app;
    app.Run();
}

When running on the target device, I get the following output:

This print statement causes the bug below!
ERROR: 'this' returned from lambda (0xbec92dd4) is NOT the same as 'this' 
(0xbec92c68) !?!?!?!?!

If I try and dereference the corrupted this, I usually get a segmentation fault, which is how I discovered the bug in the first place.

Compiler Settings

arm-poky-linux-gnueabi-g++ -march=armv7-a -marm -mfpu=neon -std=c++14 \
-mfloat-abi=hard -mcpu=cortex-a9 \
--sysroot=/home/ghunter/sysroots/cortexa9hf-neon-poky-linux-gnueabi \
-O2 -pipe -g -feliminate-unused-debug-types

Linker Settings

arm-poky-linux-gnueabi-ld \
--sysroot=/home/ghunter/sysroots/cortexa9hf-neon-poky-linux-gnueabi \
-Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed

Compiler Version

~$ arm-poky-linux-gnueabi-g++ --version

arm-poky-linux-gnueabi-g++ (GCC) 6.2.0
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Could this be a compiler bug?

This seems to be a compiler bug in gcc 6.2, see:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77686

Workarounds:

Use -fno-schedule-insns2 flag (as pointed out by gbmhunter, see comment below).
Do not use -O2 optimizations or higher.

Sounds like the following compiler bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77933 (which only effects code generated with O1 optimizations or higher).

来源：https://stackoverflow.com/questions/44830566/this-captured-by-lambda-is-incorrect-gcc-compiler-bug

标签

c++