C++ templates to make several version of function with different constant

Question

Hello

Can I use template to create several instantiations of some function, different only in some constant parameter? The number of alternatives for this parameter is fixed. E.g.

I want not to rewrite (where upper is in 1..32 in powers of two)

funct(param, int upper)
 {
  some_loops(..)
     some_heavy_code_fast_for_const_and_slow_for_variable(upper)
 }

into a set of

funct_with_upper_is_1(param) // upper =1
 { manually_copied_code...heavy(1) }
funct_with_upper_is_2(param) // upper =2
 { manually_copied_code...heavy(2) }
funct_with_upper_is_4(param) // upper =4
 { manually_copied_code...heavy(4) }
funct_with_upper_is_8(param) // upper =8
 { manually_copied_code...heavy(8) }

but into templated

template<int upper>
 funct_with_fixed_upper(param)
 { the_original_code....heavy(upper) }

and then

template<upper=1> funct_with_fixed_upper(param);
template<upper=2> funct_with_fixed_upper(param);
template<upper=4> funct_with_fixed_upper(param);
template<upper=8> funct_with_fixed_upper(param);

Is this possible with C++ tempaltes?

== Verbose mode on ==

I have a lot C++ files with code like that

function_typical(long long double ***A, long long double ***B, int const_1, int const_2)
// the type "long long double" here is very correct, this is extension of compiler
{

   for(int i=1;i<100000-1;i++)
      for(int j=1;j<100000-1;j++)
         for(int k=0;k<const_1;k++)
            for(int l=k;l<const_2;l++) {
                // some cray work with array like
                A[i][j][l-k]+=(B[i][j][l-k]+A[i+1][j][l-k]+A[i][j+1][l-k]-A[i-1][j][k]-A[i][j-1][l-k]/2.2)/88.3;
                if(A[i][j][l-k]>sin_lld(A[i][j-1][l-k])){B[i][j][l-k]=A[i][j][k]*4;}
            }
 }

This is just an example, but:

• I can't interchange the loops;
• the 2 outer loops, i & j have a lot of iterations
• the 2 inner (nested), k& l have a bit of iterations, number of which is passed into the function_typical and the set of them are fixed, e.g. const_1 and const_2 is one of pairs: (2,3), (4,5), (3,5). Total number of allowed pairs is smaller then 10.

The problem with this code is its speed is very low. If I will fix const_1 and const_2 in this code to numberic constants, compiler will do a great job in optimizing (e.g unrolling all k and all l iterations, doing a some smart job).

But I physically can't change every typical-like function of every set of (const_1 and const_2) pair. Also, constant propagator of compiler can't propagate constants of the set info the function (this is a networking server and the client does a selection of some const_1 and const_2 pair form fixed set).

So I know in compile-time all the alternatives of pairs. But I have no chance of rewrite every function by hand.

== verbose mode off ==

Thanks in advance

Answer 1

Absolutely, this is entirely possible. If you take an int by template, it is valid wherever a constant expression is valid.

template<int const_1, int const_2> function_typical(long long double ***A, long long double ***B)
// the type "long long double" here is very correct, this is extension of compiler
{

   for(int i=1;i<100000-1;i++)
      for(int j=1;j<100000-1;j++)
         for(int k=0;k<const_1;k++)
            for(int l=k;l<const_2;l++) {
                // some cray work with array like
                A[i][j][l-k]+=(B[i][j][l-k]+A[i+1][j][l-k]+A[i][j+1][l-k]-A[i-1][j][k]-A[i][j-1][l-k]/2.2)/88.3;
                if(A[i][j][l-k]>sin_lld(A[i][j][l-k])){B[i][j][l-k]=A[i][j][k]*4;}
            }
 }

This should re-compile straight, but you'll have to alter the call sites. Also, don't forget that templated code has to have the full source in all translation units and can't be defined in just one.

Answer 2

Provided your original template:

template<int upper> 
  funct_with_fixed_upper(param)
  { the_original_code....heavy(upper) }

then when you call it, you would do so like this:

 funct_with_fixed_upper<1>(param);
 funct_with_fixed_upper<2>(param);`

if you need to specialize any of them based on the constant you would do it like this:

 template<> funct_with_fixed_upper<1>(param) { // code here };

and as others have already said, this would simplifiy your code maintence but would not really reduce the size of code compiled as the compiler would still expand this out...