What does new(3) mean?

Question

SelectInst *Sel = new(3) SelectInst(C, S1, S2, NameStr, InsertBefore);

What's the meaning of the new operator here?

What's the meaning of the number 3 after the new operator?

Answer 1

This code comes from LLVM's codebase. There's a custom operator new in scope and it is being used to placement-new initialize the objects (cfr. placement syntax)

void *User::operator new(size_t Size, unsigned Us) {
  return allocateFixedOperandUser(Size, Us, 0);
}

Here's a toy example:

class SelectInst
{
public:
  int x;
};

void *operator new(size_t Size, unsigned Us) {
                                ^^^^^^^^^^^ 3 is passed here
                   ^^^^^^^^^^^ allocation size requested

  return ... // Allocates enough space for Size and for Us uses
}

SelectInst *Create() {
  return new(3) SelectInst();
}

int main()
{
  auto ptr = Create();
  return 0;
}

Live Example

Specifically that code is being used to adjust the allocated space to fit additional data.