C++: Crash when compiling with gcc 9.1, 9.2 with any level of optimization - crash

Question

The following code crashes if compiled with gcc.9.1 and 9.2. If compiled with older gcc everything is ok.
#include <cstdlib>
struct A
{
A()
{
set( nullptr );
}
std::ptrdiff_t offset_;
void set( void* ptr )
{
offset_ = reinterpret_cast<char*>( ptr ) - reinterpret_cast<char*>( this );
}
void* get()
{
return reinterpret_cast<char*>( this ) + offset_;
}
};
int main()
{
A a;
int *b = static_cast<int*>( a.get() );
if( !b )
a.set( b = new int{ 10 } );
return *b;
}
Link to the code on the coliru: http://coliru.stacked-crooked.com/a/3f5b4623a98fd0b5
Looking to the assembler I see that gcc eliminates the if statement. Link to the godbolt: https://godbolt.org/z/HrX3Uc
So the question is:
Is it a bug in the gcc which needs to be reported or my program contains some UB ?

Answer 1

struct A
{
A()
{
set( this );
}
std::ptrdiff_t offset_;
void set( void* ptr )
{
offset_ = reinterpret_cast<char*>( this ) - reinterpret_cast<char*>( ptr );
}
bool hasValue()
{
return offset_ != 0;
}
void* get()
{
return reinterpret_cast<char*>( this ) - offset_;
}
};
int main()
{
A a;
int *b = static_cast<int*>( a.get() );
// nullptr != 0
if(!a.hasValue())
a.set( b = new int{ 10 } );
return *b;
}