Re: Possible optimization opportunity when assigning to a member in the constructor

[deadalnix]

On Friday, 15 March 2013 at 04:09:25 UTC, Ali Çehreli wrote:
struct Inner below has an opAssign that gets called in 
Outer.this even with -O. That opAssign call seems unnecessary:

import std.stdio;

struct Inner
{
    int i;

    void opAssign(Inner rhs)
    {
        writeln("moving");
    }
}

struct Outer
{
    Inner inner;

    this(int i)
    {
        writeln("Assigning to this.inner");
        this.inner = Inner(i);
    }
}

void main()
{
    writeln("Constructing main.inner");
    auto inner = Inner(42);

    writeln("Constructing main.outer");
    auto outer = Outer(43);
}

The two lines in main are different from the point of view of 
two Inners in the program: The first one is the construction of 
an Inner variable. The second one is the construction of an 
Outer variable which happens to have an Inner member.

Here is the output of the program:

  Constructing main.inner
  Constructing main.outer
  Assigning to this.inner
  moving

I think the last line should not happen.

Naturally, there is no opAssign called when constructing 
main.inner. However, the assignment to this.inner in the 
constructor is treated as a "move" of the value Inner(i) on top 
of the value of Inner.init. Although there is nothing wrong 
with that, I think the compiler can simply blit Inner(i) to 
this.inner and doing so would be safe.

Does that make sense? Something like "If an rvalue is assigned 
to a member in a constructor, do not call opAssign() of the 
member".

Ali

Yes, in general, first assignment in a constructor should be 
considered as a declaration if the value is not read before.

This allow for the mentioned optimization, but many very other 
important stuff as well, like the construction of immutable 
objects.