Align a variable on the stack.

[TheFlyingFiddle via Digitalmars-d-learn]

On Thursday, 5 November 2015 at 11:14:50 UTC, Marc Schütz wrote:
> On Thursday, 5 November 2015 at 03:52:47 UTC, TheFlyingFiddle 
> wrote:
> Can you publish two compilable and runnable versions of the 
> code that exhibit the difference? Then we can have a look at 
> the generated assembly. If there's really different code being 
> generated depending on whether the .init value is explicitly 
> set to float.nan or not, then this suggests there is a bug in 
> DMD.

I created a simple example here:

struct A { float x, y, z ,w; }
struct B
{
    float x=float.nan;
    float y=float.nan;
    float z=float.nan;
    float w=float.nan;
}

void initVal(T)(ref T t, ref float k)
{
     pragma(inline, false);
     t.x = k;
     t.y = k * 2;
     t.z = k / 2;
     t.w = k^^3;
}


__gshared A[] a;
void benchA()
{
     A val;
     foreach(float f; 0 .. 1000_000)
     {
	val = A.init;
	initVal(val, f);
	a ~= val;
     }
}

__gshared B[] b;
void benchB()
{
     B val;
     foreach(float f; 0 .. 1000_000)
     {
         val = B.init;
         initVal(val, f);
	b ~= val;
     }
}


int main(string[] argv)
{
     import std.datetime;
     import std.stdio;

     auto res = benchmark!(benchA, benchB)(1);
     writeln("Default: ", res[0]);
     writeln("Explicit: ", res[1]);
	
     return 0;
}

output:

Default:  TickDuration(1637842)
Explicit: TickDuration(167088)

~10x slowdown...