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List:       gcc
Subject:    Singletons and optimisation
From:       Carlo Wood <carlo () runaway ! xs4all ! nl>
Date:       1999-09-19 17:12:41
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Hi,

has been some time I mailed here :).

I am trying to figure out a way to optimise

Singleton::instance()->method();

where `method' is not a static method, but doesn't
use `this' for some reason.

Marking Singleton::instance() with __attribute__ ((const))
should help imho, but it doesn't.

Example:

class Singleton {
public:
  static Singleton *instance(void) __attribute__ ((const));
  int add(int i) const { return i + 16; }
private:
  Singleton(void) { }
  static Singleton *instance_;
};

Singleton *Singleton::instance_;

Singleton *Singleton::instance(void)
{ 
  if (!instance_)
    instance_ = new Singleton;
  return instance_;
}

int main(void)
{ 
  return Singleton::instance()->add(1);
}

Does NOT have to call Singleton::instance()
(which is marked as __attribute__ ((const))!)
and could return immedeately 17.

But instead egcs (g++ -O6 test.cc) produces:

main:
.LFB1:
        pushl %ebp
.LCFI0:
        movl %esp,%ebp
.LCFI1:
        cmpl $0,_9Singleton.instance_		}
        jne .L23				}
        pushl $1				} Redundant code
.LCFI2:						}
        call __builtin_new			}
        movl %eax,_9Singleton.instance_	}
.L23:
        movl $17,%eax
        leave
        ret

and hence is initializing `instance_' (0x804997c).

[ Note: Making `add' also static is not an option, `add' is actually
part of a base class that is also used for non-singletons and CAN
access non-static member functions since it depends on template
parameters. ]

-- 
 Carlo Wood  <carlo@runaway.xs4all.nl>

PS Please CC me when replying to the list!

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