42

Yesterday night, at the monthly NWCPP meeting Walter Bright gave a presentation on meta-programming using the D language. Once again, D put C++ to shame.

Because of transportation arrangements I could not accompany Walter et. Co to the watering hole after the lecture. Instead I went home and decided to test how my D.NET work-in-progress compiler handles templates, and what kind of code it generates.

I picked a variadic template for my test, which computes the maximum of an arbitrarily long list of numbers (adapted from a version written by Andrei Alexandrescu) :

import System;

auto max(T1, T2, Tail...)(T1 first, T2 second, Tail args)
{
 auto r = second > first ? second : first;
 static if (Tail.length == 0) {
   return r;
 }
 else {
   return max(r, args);
 }
}

void main()
{
  uint k = 42;
  auto i = max(3, 2, k, 2.5);
  Console.WriteLine(i);
}

The program above prints 42 (of course), and here's how the generated IL looks like:

//--------------------------------------------------------------
// max.d compiled: Thu Jan 22 19:38:26 2009
//--------------------------------------------------------------
.assembly extern mscorlib {}
.assembly extern dnetlib {}
.assembly 'max' {}

.module 'max'

//--------------------------------------------------------------
// main program
//--------------------------------------------------------------
.method public hidebysig static void _Dmain ()
{
.entrypoint
.maxstack 4
.locals init (
[0] unsigned int32 'k',
[1] float64 'i'
)
ldc.i4 42
stloc.s 0    // 'k'
ldc.i4 3
ldc.i4 2
ldloc.0    // 'k'
ldc.r8 2.5
call float64 _D3max16__T3maxTiTiTkTdZ3maxFiikdZd (
  int32 'first', int32 'second', unsigned int32, float64)
stloc.s 1    // 'i'
ldloc.1    // 'i'
call void [mscorlib]System.Console::'WriteLine' (float64)
ret
}
.method public hidebysig static float64 _D3max16__T3maxTiTiTkTdZ3maxFiikdZd (
 int32 'first', int32 'second', unsigned int32, float64)
{
.maxstack 4
.locals init (
[0] int32 'r'
)
ldarg.1    // 'second'
ldarg.0    // 'first'
bgt L0_max
ldarg.0    // 'first'
br L1_max
L0_max:
ldarg.1    // 'second'
L1_max:
stloc.s 0    // 'r'
ldloc.0    // 'r'
ldarg.2    // '_args_field_0'
ldarg.3    // '_args_field_1'
call float64 _D3max14__T3maxTiTkTdZ3maxFikdZd (
int32 'first', unsigned int32 'second', float64)
ret
}
.method public hidebysig static float64 _D3max14__T3maxTiTkTdZ3maxFikdZd (
 int32 'first', unsigned int32 'second', float64)
{
.maxstack 3
.locals init (
[0] unsigned int32 'r'
)
ldarg.1    // 'second'
ldarg.0    // 'first'
conv.u4
bgt L2_max
ldarg.0    // 'first'
conv.u4
br L3_max
L2_max:
ldarg.1    // 'second'
L3_max:
stloc.s 0    // 'r'
ldloc.0    // 'r'
ldarg.2    // '_args_field_0'
call float64 _D3max12__T3maxTkTdZ3maxFkdZd (unsigned int32 'first', float64 'second')
ret
}
.method public hidebysig static float64 _D3max12__T3maxTkTdZ3maxFkdZd (
 unsigned int32 'first', float64 'second')
{
.maxstack 2
.locals init (
[0] float64 'r'
)
ldarg.1    // 'second'
ldarg.0    // 'first'
conv.r8
bgt L4_max
ldarg.0    // 'first'
conv.r8
br L5_max
L4_max:
ldarg.1    // 'second'
L5_max:
stloc.s 0    // 'r'
ldloc.0    // 'r'
ret
}

Edit: One more reason for loving D templates: pasting D code into HTML does not require replacing angular brackets with < and > respectively!