On Mon, 16 Dec 2002, Mark Mitchell wrote:
--On Monday, December 16, 2002 04:55:52 PM +0100 Richard Guenther
<address@hidden> 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.