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c#performancedebuggingrelease

Debug vs. Release performance

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".

Solution

  • 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.