Question

Are there different .net CLRs for x86 and x64 ?

decimal i = 89;
i=i++;

Will above code have different native code generated by CLR on x86, x64 ?

UPDATE:
If yes then when i install visual studio on my machine will install both the CLR versions independent of machine bits ? Has it to do something with performance ?

Answer 1

Yes, there is both a 32-bit and a 64-bit specific JITter. Which one your code will run under depends both on the host environment/process that loaded the CLR and the assembly itself.

When you build your assemblies you can choose whether it will target x86, x64, or “AnyCPU”.

• AnyCPU will run as 32-bit under 32-bit and 64-bit under 64-bit.
• 32-bit will run as 32-bit on 32-bit and will run under WOW64 on 64-bit.
• 64-bit won’t run under 32-bit, but will run as 64-bit on 64-bit.

There is also a case for ‘legacy’ assemblies which were built before this facility was introduced. I believe they act the same as the 32-bit ones.

Note that if your assembly has any 32-bit native dependencies then you should also mark it as 32-bit, otherwise it will be loaded as 64-bit in a 64-bit environment and the dependencies will fail to load.

To answer your updated questions:

64-bit machines will have both the 32-bit and the 64-bit CLR installed. The 32-bit is needed to run 32-bit assemblies under WOW64. (For example, Visual Studio is 32-bit!)

32-bit can sometimes run faster because pointers are smaller, so there is less cache/memory wasted on them. On the other hand, 64-bit mode has more registers available for the JITter to use. It depends on the characteristics of the code.