A scenario in which clang will generate ud2 instruction while gcc not

Check following main.cpp:

# cat main.cpp
#include <functional>
#include <thread>

int main()
{
    int a = 2;

    std::function<void ()> work = [&]()
    {
        if (a < 1) {
            return 1;
        }
        while (1)
        {
            if (a > 1)
            {
                break;
            }
        }
    };


    std::thread t(mergeWork);
    t.join();
    return 0;
}

The prototype of work is void (), while in a < 1 branch, it actually has a return value:

if (a < 1) {
    return 1;
}

Compile the program with g++, and execute it:

# g++ -g -pthread main.cpp
# ./a.out
#

It runs smoothly. Switch to clang++:

# clang++ -g -pthread main.cpp
main.cpp:20:5: warning: control may reach end of non-void lambda [-Wreturn-type]
    };
    ^
1 warning generated.
# ./a.out
Illegal instruction (core dumped)

A warning is generated during building and “Illegal instruction” occurs during running. Check the assembly code of work:

 disassemble
Dump of assembler code for function main::$_0::operator()() const:
   0x0000555555555100 <+0>:     push   %rbp
   0x0000555555555101 <+1>:     mov    %rsp,%rbp
   0x0000555555555104 <+4>:     mov    %rdi,-0x8(%rbp)
   0x0000555555555108 <+8>:     mov    -0x8(%rbp),%rdi
=> 0x000055555555510c <+12>:    mov    (%rdi),%rax
   0x000055555555510f <+15>:    cmpl   $0x1,(%rax)
   0x0000555555555112 <+18>:    mov    %rdi,-0x10(%rbp)
   0x0000555555555116 <+22>:    jge    0x555555555123 <main::$_0::operator()() const+35>
   0x000055555555511c <+28>:    mov    $0x1,%eax
   0x0000555555555121 <+33>:    pop    %rbp
   0x0000555555555122 <+34>:    retq
   0x0000555555555123 <+35>:    jmpq   0x555555555128 <main::$_0::operator()() const+40>
   0x0000555555555128 <+40>:    mov    -0x10(%rbp),%rax
   0x000055555555512c <+44>:    mov    (%rax),%rcx
   0x000055555555512f <+47>:    cmpl   $0x1,(%rcx)
   0x0000555555555132 <+50>:    jle    0x55555555513d <main::$_0::operator()() const+61>
   0x0000555555555138 <+56>:    jmpq   0x555555555142 <main::$_0::operator()() const+66>
   0x000055555555513d <+61>:    jmpq   0x555555555128 <main::$_0::operator()() const+40>
   0x0000555555555142 <+66>:    ud2
End of assembler dump.

We can find an ud2 instruction at the end of lambda function. Modify the a < 1 branch:

if (a < 1) {
     return;
 }

This time, the program runs OK.