Find on sorted range slower?

[John Colvin via Digitalmars-d-learn]

On Friday, 7 August 2015 at 00:35:58 UTC, Tofu Ninja wrote:
> void main()
> {
> 	auto a = new int[100*1024*1024];
> 	for(int i = 0; i < 100*1024*1024; i++)
> 	{
> 		a[i] = i;
> 	}
>
> 	enum f = 100*1024*1000;
>
> 	StopWatch sw;
> 	{
> 		sw.start();
> 		auto temp = assumeSorted(a).find(f);
> 		sw.stop();
> 	}
> 	auto t1 = sw.peek();
>
> 	sw.reset();
>
> 	{
> 		sw.start();
> 		auto temp = a.find(f);
> 		sw.stop();
> 	}
> 	auto t2 = sw.peek();
>
> 	writeln("Sorted\t", t1.length);
> 	writeln("Regular\t", t2.length);
> 	writeln("Ratio\t", float(t1.length)/ float(t2.length));
> }
>
>
> I am getting the assumeSorted version to be about 3x slower 
> than the regular find, that seems very counter intuitive. 
> Anyone know why this would be, seems like a very odd thing to 
> happen. I expected the assumeSorted to be faster, expect it to 
> do a binary search, instead if a linear one.

As usual, which compiler, which compiler version, which 
compilation flags?