[Bug c++/68922] New: g++ fails to generate code for catch clause with specific optimizations enabled

["alban.lefebvre at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68922

            Bug ID: 68922
           Summary: g++ fails to generate code for catch clause with
                    specific optimizations enabled
           Product: gcc
           Version: 5.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: alban.lefebvre at gmail dot com
  Target Milestone: ---

Hello,

The following code fails to generate a try/catch clause on g++ 5.2.1, 4.9.2,
4.8.4, 4.8.2:

#include <iostream>
#include <exception>

class Base
{
public:
    virtual ~Base() {}
};

class C1 : public virtual Base
{
};

class C2 : public virtual Base
{
public:
    virtual void foo() = 0;
};

class D : public C1, public C2
{
public:
    virtual void foo() 
    {
        throw std::exception();
    }
};

int main()
{
    C2 * c2 = new D();

    try
    {
        c2->foo();
    }
    catch (...)
    {
        std::cout << "Caught some exception" << std::endl;
    }

    return 0;
}

when compiled with O2 optimization

  g++ main.cpp -Wall -Wextra -pedantic -O2 && ./a.out

I get the following error when executing it:

terminate called after throwing an instance of 'std::exception' 
  what():  std::exception
Aborted (core dumped)

It seems that __cxa_begin_catch, __cxa_end_catch calls do not get generated:

main:
    sub    $0x8,%rsp
    mov    $0x10,%edi
    callq  0x400850 <_Znwm@plt>
    lea    0x8(%rax),%rdi
    movq   $0x400ea0,(%rax)
    movq   $0x400ed8,0x8(%rax)
    callq  0x400b10 <_ZThn8_N1D3fooEv>
    xor    %eax,%eax
    add    $0x8,%rsp
    retq   

It looks similar but possibly not the same as
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68184
When we remove the virtual inheritance (by removing Base), the bug doesn't
occur anymore.

As far as which flags seem to trigger the issue, here what I've tried:
-O1                       => OK 
-O2                       => BUG 
-O3                       => BUG
-O1 -ftree-pre -ftree-vrp => BUG 
-ftree-pre -ftree-vrp     => OK

Thank you