What's the purpose of the CIL nop opcode?

assembly bytecode cil
Dan Goldstein picture Dan Goldstein · Oct 24, 2008 · Viewed 47.8k times · Source

I'm going through MSIL and noticing there are a lot of nop instructions in the MSIL.

The MSDN article says they take no action and are used to fill space if the opcode is patched. They're used a lot more in debug builds than release builds.

I know that these kinds of statements are used in assembly languages to align later instructions, but why are MSIL nops needed in MSIL?

(Editor's note: the accepted answer is about machine-code NOPs, not MSIL/CIL NOPs which the question originally asked about.)

Answer

Anthony Williams picture Anthony Williams · Oct 24, 2008

NOPs serve several purposes:

  • They allow the debugger to place a breakpoint on a line even if it is combined with others in the generated code.
  • It allows the loader to patch a jump with a different-sized target offset.
  • It allows a block of code to be aligned at a particular boundary, which can be good for caching.
  • It allows for incremental linking to overwrite chunks of code with a call to a new section without having to worry about the overall function changing size.

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