Huge performance difference (26x faster) when compiling for 32 and 64 bits

Question

I was trying to measure the difference of using a for and a foreach when accessing lists of value types and reference types.

I used the fol

Answer 1

There may be several reasons why this is executing faster in 64bit on your machine. The reason I asked which CPU you were using was because when 64bit CPUs first made their appearance, AMD and Intel had different mechanisms to handle 64bit code.

Processor architecture:

Intel's CPU architecture was purely 64bit. In order to execute 32bit code, the 32bit instructions needed to be converted (inside the CPU) to 64bit instructions before execution.

AMD's CPU architecture was to build 64bit right on top of their 32bit architecture; that is, it was essentially a 32bit architecture with 64bit extentions - there was no code conversion process.

This was obviously a few years ago now, so I've no idea if/how the technology has changed, but essentially, you would expect 64bit code to perform better on a 64bit machine since the CPU is able to work with double the amount of bits per instruction.

.NET JIT

It's argued that .NET (and other managed languages like Java) are capable of outperforming languages like C++ because of the way the JIT compiler is able to optimize your code according to your processor architecture. In this respect, you might find that the JIT compiler is utilizing something in 64bit architecture that possibly wasn't available or required a workaround when executed in 32bit.

Note:

Rather than using DoubleWrapper, have you considered using Nullable or shorthand syntax: double? - I'd be interested to see if that has any impact on your tests.

Note 2: Some people seem to be conflating my comments about 64bit architecture with IA-64. Just to clarify, in my answer, 64bit refers to x86-64 and 32bit refers to x86-32. Nothing here referenced IA-64!