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I am new to F# so not sure if this the correct group or not, but here goes.

I have a class method that adds an input parameter to an ADO.NET command object. Pretty standard stuff, this is the code. 

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member public x.AddInputParameter(pName, pType, pValue) = 

let param = command.CreateParameter()

param.Direction <- ParameterDirection.Input

param.ParameterName <- pName

param.DbType <- pType

param.Value <- pValue

command.Parameters.Add(param) |> ignore

Now my question is more to do wit the IL that is created

I have removed the middle portion of the IL because it just loads and sets the vlaues for the properties

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.method public 

instance void AddInputParameter<a> (string pName, valuetype [System.Data]System.Data.DbType pType,

!!a pValue

) cil managed 

{

    

// Method begins at RVA 0x2374

    

// Code size 67 (0x43)

.maxstack 4

.locals init ([0] class [System.Data]System.Data.SqlClient.SqlParameter param,

              [1] class [System.Data]System.Data.SqlClient.SqlParameter,

              [2] class [System.Data]System.Data.SqlClient.SqlParameter

)

IL_0000: nop

IL_0001: ldarg.0

IL_0002: ldfld class [System.Data]System.Data.SqlClient.SqlCommand FsharpAdo.AdoCommand/Command::command

IL_0015: ldarg.1

    IL_0016: callvirt instance void [System.Data]System.Data.Common.DbParameter::set_ParameterName(string)

IL_003a: call instance class [System.Data]System.Data.SqlClient.SqlParameter 

[System.Data]System.Data.SqlClient.SqlParameterCollection::Add(class [System.Data]System.Data.SqlClient.SqlParameter)

  <standard code removed>

IL_003f: stloc.1

IL_0040: ldloc.1

IL_0041: stloc.2

IL_0042: ret

} // end of method Command::AddInputParameter

Why does the function contain three locals when only one is defined and

why the last three lines of code before the return?

I can understand the need to return a value from a function but why fool around with two uninitialized locals? No where in the IL other than the last couple lines are the two extra locals used.

Thanks for you time.

.

First of all, it looks like you're compiling in debug mode, so the IL output isn't going to be optimized (partly because you may want to set breakpoints and inspect intermediate values, although I can't see why that would be a factor here). However, compiling in release mode only removes one of the locals, and I'm not sure why the other one is kept around (I suspect that the call to ignore isn't being completely optimized away).

In general is there a reason (other than curiosity) that you're concerned about the generated IL? I would be shocked if this had any actual performance impact (and in fact the JITter may be able to remove write-only locals, so the generated machine code might not even be affected).

By on 6/28/2011 8:08 AM ()

Apparently I have misunderstood what the ignore statement does or how it should be used.

if I change the AddInputParameter function by removing the |> ignore then the code generated does not contain the additional locals. Really do not understand that one.

But then I need to add the |> ignore to the calls to the function or I get a warning from the compiler.

How could, or can, the function be written so the |> ignore statement is not needed?

By on 7/3/2011 6:05 AM ()

Hi Dennis,

I think you should just use ignore within the function. The fact that the resulting code uses a local is an unexpected code generation quirk, but it seems extremely unlikely to present any real world performance degradation.

-Keith

By on 7/3/2011 6:48 AM ()

Thanks for the response.

Yes I was in debug mode, mainly for a couple other methods but I happened to notice this one and it is small and simple enough for an example.

Anyway, changing to release mode resulted in the same IL.

Not a performance issue, just curios.

I assume the the JIT can optimize the unused locals out, does it? I have no idea.

Thanks again.

By on 6/28/2011 10:08 AM ()

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