On Friday, 20 September 2013 at 10:29:24 UTC, andrea9940 wrote:
On Friday, 20 September 2013 at 09:44:51 UTC, Namespace wrote:
This prints 'ref' if you change func(A a) to func(const A a)
the match of const ref isn't prefered over A a because const
need an implicit conversion.
Thanks for the tip.
a + b is an rvalue and will moved to func(A a). This is even
faster than ref. ;)
A() is also moved.
Mmm... here is the assembly generated (with -release -O), it
seems to me that the struct is always copied.
; fun(a + b)
LEA EAX,[EBP-604]
PUSH EAX
LEA EAX,[EBP-404]
PUSH EAX
LEA EAX,[EBP-804]
CALL 00402010 ;opAdd()
MOV EBX,EAX ;pointer to the stack
MOV ECX,80
ADD EBX,1FC
004020C3: PUSH DWORD PTR DS:[EBX] ; copy !
SUB EBX,4 ; copy !
LOOP SHORT 004020C3 ; copy !
CALL 00402048 ; fun(const A a)
;fun(A())
PUSH 80
MOV EAX,OFFSET 00422930
PUSH EAX
CALL 00405DB0
MOV DWORD PTR SS:[EBP-4],EAX
MOV ECX,DWORD PTR SS:[EBP-4]
XOR EAX,EAX
MOV DWORD PTR DS:[ECX],EAX
MOV EAX,DWORD PTR SS:[EBP-4]
MOV DWORD PTR DS:[EAX+4],0
; a lot of movs later ...
MOV EAX,DWORD PTR SS:[EBP-4]
MOV DWORD PTR DS:[EAX+1FC],0
MOV ESI,DWORD PTR SS:[EBP-4]
LEA EDI,[EBP-204]
MOV ECX,80
REP MOVS DWORD PTR ES:[EDI],DWORD PTR DS:[ESI]
ADD ESP,8
; ^ a lot of istructions to set a struct to zero imo
LEA EBX,[EBP-8]
MOV ECX,80
0040271B: PUSH DWORD PTR DS:[EBX] ; copy !
SUB EBX,4 ; copy !
LOOP SHORT 0040271B ; copy !
CALL 00402048 ;fun(const A a)
; fun(a)
LEA EAX,[EBP-804] ;load the reference
CALL 00402060 ; fun(const ref A a)
If you have doubt about the assembly, please feeld free to open a
bug report.
And for performance:
----
import std.stdio;
struct A {
public:
uint id;
this(uint id) {
this.id = id;
writeln("CTor A with ", id);
}
this(this) {
writeln("Postblit A with ", this.id);
}
~this() {
writeln("DTor A with ", this.id);
}
A opBinary(string op : "+")(ref const A a) {
return A(this.id + a.id);
}
}
void func(const A a) {
writeln("Value call with A::", a.id);
}
void func(ref const A a) {
writeln("Ref call with A::", a.id);
}
void main()
{
A a = A(42);
A b = A(23);
func(a + b);
func(A(1337));
func(a);
}
----
Output:
----
CTor A with 42
CTor A with 23
CTor A with 65
Value call with A::65
DTor A with 65
CTor A with 1337
Value call with A::1337
DTor A with 1337
Ref call with A::42
DTor A with 23
DTor A with 42
----
No Postblit call.
