Question

I heard compiling in Release mode generates optimized code than in Debug mode, which is fine.

But is this optimization in the IL? is it in the machine code once the CLR runs it? is the metadata structure different from PE compiled in Release and Debug?

thanks

Answer 1

Building in Release build turns on the /optimize compile option for the C# compiler. That has a few side-effects, the IL indeed changes but not a great deal. Notable is that the compiler no longer makes an effort to make the code perfectly debuggable. It for example skips an empty static constructor, it no longer emits the NOP opcodes that allows you to set a breakpoint on a curly brace and allows local variables with different scopes to overlap in a stack frame. Small stuff.

The most important difference is the [Debuggable] attribute that’s emitted for the assembly, its IsJITOptimizerDisabled property is false.

Which turns on the real optimizer, the one that’s built into the jitter. You’ll find the list of optimizations it performs in this answer. Do note the usefulness of this approach, any language benefits from having the code optimizer in the jitter instead of the compiler.

So in a nutshell, very minor changes in the IL, very large changes in the generated machine code.