[Issue 9732] New: Do not call opAssign() for the first assignment to member in the constructor

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=9732

           Summary: Do not call opAssign() for the first assignment to
                    member in the constructor
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: performance
          Severity: enhancement
          Priority: P3
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: acehreli at yahoo.com


--- Comment #0 from Ali Cehreli <acehreli at yahoo.com> 2013-03-15 17:16:46 PDT ---
This is about struct members of both structs and classes.

It is a common idiom in constructors to assign rvalues to members:

    this(int i)
    {
        this.inner = Inner(i);    // <-- assign from rvalue
    }

If 'Inner' above is a struct that has an rvalue opAssign() defined, currently
dmd compiles a call to that opAssign(). However, it is safe to elide that call
for the first such assignment in the constructor. Rather, the compiler can blit
the rvalue on top of the .init state of the member.

The following program demonstrates the two calls to opAssign() that could be
elided:

import std.stdio;

struct Inner
{
    int i;

    void opAssign(Inner rhs)
    {
        writeln("rvalue opAssign called");
    }
}

struct OuterStruct
{
    Inner inner;

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

class OuterClass
{
    Inner inner;

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

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

    writeln("\nConstructing main.outer_struct");
    auto outer_struct = OuterStruct(43);

    writeln("\nConstructing main.outer_class");
    auto outer_class = new OuterClass(44);
}

The output:

  Constructing main.inner

  Constructing main.outer_struct
  Assigning to OuterStruct.inner
  rvalue opAssign called           // <-- could be elided

  Constructing main.outer_class
  Assigning to OuterClass.inner
  rvalue opAssign called           // <-- could be elided

Note that if dmd merely blitted the rvalue, the two lines "rvalue opAssign
called" would not be printed.

Thank you,
Ali

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