[pooma-dev] Re: InlineEvaluator implementation question

[Richard Guenther]

On Mon, 16 Dec 2002, Mark Mitchell wrote:

> --On Monday, December 16, 2002 04:55:52 PM +0100 Richard Guenther
> <rguenth at tat.physik.uni-tuebingen.de> wrote:
>
> > Hi!
> >
> > Does anyone remember why we create copies of the LHS and RHS inside
> > the KernelEvaluator<InlineKernelTag>::evaluate() methods (within
> > ReductionEvaluator<InlineKernelTag>::evaluate() is similar code)? I.e.
> > there is code like
> >
> >   template<class LHS,class Op,class RHS,class Domain>
> >   inline static void evaluate(const LHS& lhs,const Op& op,const RHS& rhs,
> >                               const Domain& domain,WrappedInt<1>)
> >   {
> >     CTAssert(Domain::unitStride);
> >     PAssert(domain[0].first() == 0);
> >     LHS localLHS(lhs);
> >     RHS localRHS(rhs);
> >     int e0 = domain[0].length();
> >     for (int i0=0; i0<e0; ++i0)
> >       op(localLHS(i0),localRHS.read(i0));
> >   }
>
> I'm pretty sure that this copy allowed some C++ compilers (KCC) to see
> that some parts of lhs/rhs were loop-invariant, and then hoist references
> to those fields out of the loop.  (The compiler can see that nothing can
> modify localLHS; it's less obvious to it that nothing can modify rhs
> since it doesn't know what else might point to that location.)

Hmm - as both, lhs and rhs are declared const, isnt this enough to tell
the compiler? Or has the compiler to assume every function call can have
a side-effect on any (but local) variable?

Well, at least gcc creates worse (larger) code with copying than without.

Richard.