digitalmars.D - Possible optimization opportunity when assigning to a member in the
- =?UTF-8?B?QWxpIMOHZWhyZWxp?= (45/45) Mar 14 2013 struct Inner below has an opAssign that gets called in Outer.this even
- deadalnix (6/53) Mar 14 2013 Yes, in general, first assignment in a constructor should be
- =?UTF-8?B?QWxpIMOHZWhyZWxp?= (4/13) Mar 15 2013 Thank you! Opened an enhancement request:
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
Mar 14 2013
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". AliYes, 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.
Mar 14 2013
On 03/14/2013 09:54 PM, deadalnix wrote:On Friday, 15 March 2013 at 04:09:25 UTC, Ali Çehreli wrote:Thank you! Opened an enhancement request: http://d.puremagic.com/issues/show_bug.cgi?id=9732 AliSomething like "If an rvalue is assigned to a member in a constructor, do not call opAssign() of the member". AliYes, 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.
Mar 15 2013