“称为纯虚函数"在 gcc 4.4 上，但不在较新版本或 clang 3.4 上 [英] "pure virtual function called" on gcc 4.4 but not on newer version or clang 3.4

问题描述

我有一个 MCVE，它在使用 g++ 4.4.7 版编译时在我的一些机器上崩溃，但可以在 clang++ 3.4.2 版和 g++ 6.3 版中使用.

I've got an MCVE which, on some of my machines crashes when compiled with g++ version 4.4.7 but does work with clang++ version 3.4.2 and g++ version 6.3.

我想知道它是来自未定义的行为还是来自这个古老版本的 gcc 的实际错误.

I'd like some help to know if it comes from undefined behavior or from an actual bug of this ancient version of gcc.

#include <cstdlib>

class BaseType
{
public:
    BaseType() : _present( false ) {}
    virtual ~BaseType() {}

    virtual void clear() {}

    virtual void setString(const char* value, const char* fieldName)
    {
        _present = (*value != '');
    }

protected:
    virtual void setStrNoCheck(const char* value) = 0;

protected:
    bool _present;
};

// ----------------------------------------------------------------------------------

class TypeTextFix : public BaseType
{
public:
    virtual void clear() {}

    virtual void setString(const char* value, const char* fieldName)
    {
        clear();
        BaseType::setString(value, fieldName);
        if( _present == false ) {
            return; // commenting this return fix the crash. Yes it does!
        }
        setStrNoCheck(value);
    }

protected:
    virtual void setStrNoCheck(const char* value) {}
};

// ----------------------------------------------------------------------------------

struct Wrapper
{
    TypeTextFix _text;
};

int main()
{
    {
        Wrapper wrapped;
        wrapped._text.setString("123456789012", NULL);
    }
    // if I add a write to stdout here, it does not crash oO
    {
        Wrapper wrapped;
        wrapped._text.setString("123456789012", NULL); // without this line (or any one), the program runs just fine!
    }
}

编译 &运行

g++ -O1 -Wall -Werror thebug.cpp && ./a.out
pure virtual method called
terminate called without an active exception
Aborted (core dumped)

这实际上是最小的，如果删除此代码的任何功能，它就可以正常运行.

This is actually minimal, if one removes any feature of this code, it runs correctly.

代码片段在使用 -O0 编译时可以正常工作，BUT 在每个标志使用 -O0 +flag 编译时仍然可以正常工作 定义的 -O1GnuCC 文档.

The code snippet works fine when compiled with -O0, BUT it still works fine when compiled with -O0 +flag for every flag of -O1 as defined on GnuCC documentation.

生成一个核心转储，可以从中提取回溯:

A core dump is generated from which one can extract the backtrace:

(gdb) bt
#0  0x0000003f93e32625 in raise () from /lib64/libc.so.6
#1  0x0000003f93e33e05 in abort () from /lib64/libc.so.6
#2  0x0000003f98ebea7d in __gnu_cxx::__verbose_terminate_handler() () from /usr/lib64/libstdc++.so.6
#3  0x0000003f98ebcbd6 in ?? () from /usr/lib64/libstdc++.so.6
#4  0x0000003f98ebcc03 in std::terminate() () from /usr/lib64/libstdc++.so.6
#5  0x0000003f98ebd55f in __cxa_pure_virtual () from /usr/lib64/libstdc++.so.6
#6  0x00000000004007b6 in main ()

<小时>

请随时在评论中询问测试或详细信息.问:

---

Feel free to ask for tests or details in the comments. Asked:

• 是实际代码吗?是的！它是！一个字节一个字节.我已经检查并重新检查了.

• Is it the actual code? Yes! it is! byte for byte. I've checked and rechecked.

您使用的是哪个版本的 GnuCC?

What exact version of GnuCC du you use?

$ g++ --version
g++ (GCC) 4.4.7 20120313 (Red Hat 4.4.7-16)
Copyright (C) 2010 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.

我们能看到生成的程序集吗?是的，这里是 pastebin.com

推荐答案

这是 FSF GCC 中不存在的 Red Hat 特定错误.这不是您的代码中的问题.

This is a Red Hat-specific bug not present in FSF GCC. It is not a problem in your code.

在同时具有 CentOS 6 的 GCC 和 FSF GCC 4.4.7 的系统上，同时生成程序集列表并查看两者之间的差异，会跳出一点:

On a system with both CentOS 6's GCC, and FSF GCC 4.4.7, having both generate an assembly listing and viewing the differences between the two, one bit jumps out:

CentOS 6 的 GCC 生成

CentOS 6's GCC generates

movq $_ZTV8BaseType+16, (%rsp)

而 FSF GCC 4.4.7 生成

whereas FSF GCC 4.4.7 generates

movq $_ZTV11TypeTextFix+16, (%rsp)

换句话说，Red Hat 的 GCC 补丁之一导致它错误地设置了 vtable.这是您的 main 函数的一部分，您可以在 .L48: 之后不久在您自己的程序集清单中看到它.

In other words, one of Red Hat's GCC patches makes it set up the vtable incorrectly. This is part of your main function, you can see it in your own assembly listing shortly after .L48:.

Red Hat 为其 GCC 版本应用了许多补丁，其中一些补丁是影响代码生成的补丁.不幸的是，其中一个似乎有意想不到的副作用.

Red Hat applies many patches to its version of GCC, and some of them are patches that affect code generation. Unfortunately, one of them appears to have an unintended side effect.

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