template fiddling

[bearophile]

Dmitry Olshansky:
> Probably, this could be a starting point (BTW  In D you can write 
> templates very easy):
> 
> T dot_product(size_t N, T)(T[] a, T[] b){
>      static if (N == 1){
>          return a[0] * b[0];
>      }else{
>          return a[0] * b[0] + dot_product!(N-1)(a[1..$],b[1..$]);
>      }
> }
> 
> void main()
> {
>      int[] a = [1, 2, 3];
>      int[] b = [5, 6, 7];
>      assert( dot_product!3(a,b) == 38 );//sanity check
> }
> And I didn't checked the disassembly.

I have added another version:

import std.typetuple: TypeTuple;

T dot_product(size_t N, T)(T[] a, T[] b) {
     static if (N == 1)
         return a[0] * b[0];
     else
         return a[0] * b[0] + dot_product!(N-1)(a[1..$],b[1..$]);
}

template Iota(int stop) {
    static if (stop <= 0)
        alias TypeTuple!() Iota;
    else
        alias TypeTuple!(Iota!(stop-1), stop-1) Iota;
}

T dotProduct(T, int N)(T[N] a, T[N] b) {
    T result = 0;
    foreach (i; Iota!(N))
        result += a[i] * b[i];
    return result;
}

void main() {
    double[3] a = [1., 2., 3.];
    double[3] b = [5., 6., 7.];
    assert(dot_product!3(a, b) == 38.0);
    assert(dotProduct(a, b) == 38.0);
}


The asm (dmd 2.047 -O -release -inline):

_D6test6b22__T11dot_productVi3TdZ11dot_productFAdAdZd	comdat
		sub	ESP,038h
		mov	EDX,048h[ESP]
		mov	EAX,044h[ESP]
		push	EBX
		mov	ECX,044h[ESP]
		mov	EBX,048h[ESP]
		push	ESI
		dec	EBX
		mov	ESI,044h[ESP]
		mov	8[ESP],EBX
		dec	ESI
		mov	EAX,044h[ESP]
		mov	010h[ESP],ESI
		mov	EAX,8[ESP]
		mov	EBX,010h[ESP]
		dec	EBX
		dec	EAX
		fld	qword ptr [EDX]
		lea	EDX,8[EDX]
		fmul	qword ptr [ECX]
		lea	ECX,8[ECX]
		mov	0Ch[ESP],EDX
		mov	014h[ESP],ECX
		fld	qword ptr [EDX]
		lea	EDX,8[EDX]
		fmul	qword ptr [ECX]
		lea	ECX,8[ECX]
		fld	qword ptr [EDX]
		fmul	qword ptr [ECX]
		faddp	ST(1),ST
		faddp	ST(1),ST
		pop	ESI
		pop	EBX
		add	ESP,038h
		ret	010h

_D6test6b22__T11dot_productVk2TdZ11dot_productFAdAdZd	comdat
		sub	ESP,018h
		mov	EDX,028h[ESP]
		mov	EAX,024h[ESP]
		push	EBX
		mov	ECX,024h[ESP]
		mov	EBX,028h[ESP]
		push	ESI
		mov	ESI,024h[ESP]
		mov	EAX,024h[ESP]
		dec	ESI
		dec	EBX
		fld	qword ptr [EDX]
		lea	EDX,8[EDX]
		fmul	qword ptr [ECX]
		lea	ECX,8[ECX]
		fld	qword ptr [EDX]
		fmul	qword ptr [ECX]
		faddp	ST(1),ST
		pop	ESI
		pop	EBX
		add	ESP,018h
		ret	010h

_D6test6b22__T11dot_productVk1TdZ11dot_productFAdAdZd	comdat
		mov	EDX,010h[ESP]
		mov	EAX,0Ch[ESP]
		mov	EAX,4[ESP]
		fld	qword ptr [EDX]
		mov	EDX,8[ESP]
		fmul	qword ptr [EDX]
		ret	010h

_D6test6b21__T10dotProductTdVk3Z10dotProductFG3dG3dZd	comdat
		sub	ESP,048h
		fld	qword ptr 064h[ESP]
		fmul	qword ptr 04Ch[ESP]
		fldz
		faddp	ST(1),ST
		fstp	qword ptr [ESP]
		fld	qword ptr 06Ch[ESP]
		fld	qword ptr 074h[ESP]
		fxch	ST1
		fmul	qword ptr 054h[ESP]
		fxch	ST1
		fmul	qword ptr 05Ch[ESP]
		faddp	ST(1),ST
		fadd	qword ptr [ESP]
		fstp	qword ptr [ESP]
		fld	qword ptr [ESP]
		add	ESP,048h
		ret	030h


Bye,
bearophile