NOP in release build of F# code

Question

I am playing with F# in VS2010 beta2, and since I am new to F#, I just picked one of the common examples and went ahead and implemented a factorial function as: ``` let rec factorial n = if n <= 1 then 1 else n * factorial (n - 1);; ``` If I build this and look at the generated code in Reflector, I get the corresponding C# code: ``` public static int Factorial(int n) { if (n <= 1) return 1; return n * Factorial(n - 1); } ``` So if I compile Reflector's C# representation of the F# code, I would expect to get identical IL. However, if I compile both of these snippets in release mode and compare the generated IL, they are different (they are functionally identical, but still differs a bit). The C# implementation compiles to: ``` .method public hidebysig static int32 Factorial(int32 n) cil managed { .maxstack 8 L_0000: ldarg.0 L_0001: ldc.i4.1 L_0002: bgt.s L_0006 L_0004: ldc.i4.1 L_0005: ret L_0006: ldarg.0 L_0007: ldarg.0 L_0008: ldc.i4.1 L_0009: sub L_000a: call int32 TestApp.Program::…

Accepted Answer

The maxstack difference is due to the fact that the C# compiler compiles the first method with a «light» method body header, that is used whenever the code is small, has no exceptions and no locals. In that case, the maxstack is not specified and defaults to 8.

The F# compiler is using a «fat» method body header, and specifies the maxstack it has computed.

As for the nop, it's because you're compiling in debug mode. They always start a method body with a nop. See from fsharp/ilxgen.ml:

```
// Add a nop to make way for the first sequence point. There is always such a 
// sequence point even when zapFirstSeqPointToStart=false
do if mgbuf.cenv.generateDebugSymbols  then codebuf.Add(i_nop);
```

If I compile your factorial without debug symbols, I don't get a nop.

NOP in release build of F# code

Solution

Problem