c# - Debug vs. Release performance

Question

I've encountered the following paragraph:

“Debug vs. Release setting in the IDE when you compile your code in Visual Studio makes almost no difference to performance… the generated code is almost the same. The C# compiler doesn’t really do any optimization. The C# compiler just spits out IL… and at the runtime it’s the JITer that does all the optimization. The JITer does have a Debug/Release mode and that makes a huge difference to performance. But that doesn’t key off whether you run the Debug or Release configuration of your project, that keys off whether a debugger is attached.”

The source is here and the podcast is here.

Can someone direct me to a Microsoft article that can actually prove this?

Googling "C# debug vs release performance" mostly returns results saying "Debug has a lot of performance hit", "release is optimized", and "don't deploy debug to production".

Answer 1

Partially true. In debug mode, the compiler emits debug symbols for all variables and compiles the code as is. In release mode, some optimizations are included:

• unused variables do not get compiled at all
• some loop variables are taken out of the loop by the compiler if they are proven to be invariants
• code written under #debug directive is not included, etc.

The rest is up to the JIT.

Full list of optimizations here courtesy of Eric Lippert.