• imr1984 (5/5) Apr 15 2004 how good is dmd at optimising away the default initialisers when it is
• Phill (12/17) Apr 16 2004 critical.
• Walter (6/11) Apr 16 2004 critical.
• Manfred Nowak (19/21) Apr 19 2004 WinXP dmd 0.82

imr1984 <imr1984_member pathlink.com> writes:

how good is dmd at optimising away the default initialisers when it is
guaranteed that a variable will be initialised later than the variable
declaration? I am concerned about this because my program is very time critical.
I think it would be cool if there was an attribute for variable declarations not
to be initialised by default.

"Phill" <phill pacific.net.au> writes:

"imr1984" <imr1984_member pathlink.com> wrote in message
news:c5mttq$la8$1 digitaldaemon.com...
 how good is dmd at optimising away the default initialisers when it is
 guaranteed that a variable will be initialised later than the variable
 declaration? I am concerned about this because my program is very time

critical.
 I think it would be cool if there was an attribute for variable

declarations not
 to be initialised by default.

But if they arent given a default value, dont they
then just contain garbage anyway? If so, isnt it better to have a default
value?

Phill.




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"Walter" <walter digitalmars.com> writes:

"imr1984" <imr1984_member pathlink.com> wrote in message
news:c5mttq$la8$1 digitaldaemon.com...
 how good is dmd at optimising away the default initialisers when it is
 guaranteed that a variable will be initialised later than the variable
 declaration? I am concerned about this because my program is very time

critical.
 I think it would be cool if there was an attribute for variable

declarations not
 to be initialised by default.

The compiler uses 'data flow analysis' to detect and remove dead
assignments, and it works reasonably well. Try it and see.

Manfred Nowak <svv1999 hotmail.com> writes:

Walter wrote:

 The compiler uses 'data flow analysis' to detect and remove dead
 assignments, and it works reasonably well. Try it and see.

WinXP dmd 0.82

<code>
import std.c.windows.windows;
void main(){
  double dtime(){
    return (cast(double) GetTickCount) / 1000;
  }
  const long size=1_400_000_000;
  double start= dtime;
  int[] arr=new int[size/4];
  //arr[1]=1;
  double end= dtime;
  printf("%20.20f\n", end - start);
}
</code>

Although I commented the access out, the runtime stays by more than 1.5
seconds, whereas the according C program hushes by.

So long!