Re: Deterministic functions

From:
alvaro.segura.SPAMFREE@gmail.com (Alvaro Segura)
Newsgroups:
comp.std.c++
Date:
Tue, 27 Mar 2007 04:23:48 GMT
Message-ID:
<eu9nga$8d2$1@aioe.org>
Dave Harris escribi=F3:

alvaro.segura.SPAMFREE@gmail.com (Alvaro Segura) wrote (abridged):

I propose to have a special keyword to mark functions that will return
the same value if the same arguments are given AND if only functions o=

f

a special kind (probably const) are called on the object.

  class Container {
    public:
      int size() const deterministic;
      Item& getAt(index i) const deterministic;
      void add(Item item);
  };

 
How would you deal with aliasing?
 
    static Container container;
    
    void proc( const Container &c ) {
        for (int i = 0; i < c.size(); ++i)
            container.add( c.getAt(i) );
    }
 
Should the compiler assume that c is not a reference to container? Or m=

ust

it suppress the optimisation? Or is this undefined behaviour?
 
-- Dave Harris, Nottingham, UK.


Oh! Got me.
I guess it should not optimize just in case they are the same. Or how do
compilers deal with other aliasing problems? IIRC there are flags like
"assume-no-aliasing" in some compilers to make them optimize things they
could not otherwise.

The optimization (caching) should be done only if no non-const functions
are called on *that* object. If there is a call to a non-const and it
can't be sure if they are the same or a different object, then it should
make the function call again normally.

I think there are currently similar conditions in the autovectorization
optimization in GCC, where possible aliasing disables lots of potential
optimizations.

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