Monday, June 4, 2007

MISL to C# (Sharp) -> Iff (if and only if)

Today I’ll talk about the most basic and useful if-then structure. The compiler generates a conditional branch. We create a block of instructions for the structure. The block is separated by the branch instruction (like brtrue) and the branch label (like IL_0019 - where the code jumps). And our condition is on the stack. If we find a true condition branch we negate it and put it as a if statement condition. The block is initially a block of MSIL that we will convert to C# code later (
may need recursion here??). Please note that the labels are not stored in MSIL. It is just the byte offset of the MSIL in a method.

If you have not already read you are requested to read my previous post.


We take a simple method to test our theory.

public int IfStructure(int a, int b)
{
bool CS_4_0001;
if (a < b)
{
System.Console.Write("Condition is true");
}
return b;
}

Here is the MSIL code generated by Visual Studio 2005 compiler.

.method public hidebysig instance int32 IfStructure(int32 a, int32 b) cil managed
{
// Code size 31 (0x1f)
.maxstack 2
.locals init ([0] int32 CS$1$0000,
[1] bool CS$4$0001)
IL_0000: nop
IL_0001: ldarg.1
IL_0002: ldarg.2
IL_0003: clt
IL_0005: ldc.i4.0
IL_0006: ceq
IL_0008: stloc.1
IL_0009: ldloc.1
IL_000a: brtrue.s IL_0019

IL_000c: nop
IL_000d: ldstr "Condition is true"
IL_0012: call void [mscorlib]System.Console::Write(string)
IL_0017: nop
IL_0018: nop

IL_0019: ldarg.2
IL_001a: stloc.0
IL_001b: br.s IL_001d
IL_001d: ldloc.0
IL_001e: ret
} // end of method ControlStructures::IfStructure

We now start parsing:
-------------------------------------------------------------------------------------
.method public hidebysig instance int32 IfStructure(int32 a, int32 b) cil managed
{
// Code size 31 (0x1f)
.maxstack 2
.locals init ([0] int32 CS$1$0000,
[1] bool CS$4$0001)
--

These lines generate output that we do without any processing of MSIL. There is method definition and local variables. We change the local variable names to C# current names without conflict. For simplicity here we just replace '$' with '_'. One thing to evaluate the MSIL instructions we must keep a map of local variables with variable number. In this example CS$1$0000 is local variable 0 of type int. For clarity we do not show the map here. A simple STL map should work.

Output:
public int IfStructure(int a, int b)
{
int32 CS_1_0000;
bool CS_4_0001;

Stack: [Empty]

-------------------------------------------------------------------------------------
IL_0000: nop
IL_0001: ldarg.1
IL_0002: ldarg.2
--
So we push method argument 1 and 2 to stack:

Output: [None]

Stack:
a,b

-------------------------------------------------------------------------------------
IL_0003: clt
--

This instructs us if stack top-1 is less than stack top. The two elements are popped from stack and result goes to stack. No output of course.

Output: [None]

Stack:
a<b

-------------------------------------------------------------------------------------
IL_0005: ldc.i4.0
--

Load (aka push) constant integer of value 0 on stack.

Output: [None]
Stack: a<b,0
-------------------------------------------------------------------------------------
IL_0006: ceq
--

Check if stack top-1 equals stack top. Result goes to stack.
Output: [None]
Stack: a < b == 0

-------------------------------------------------------------------------------------
IL_0008: stloc.1
IL_0009: ldloc.1
--

What else? Store stack top in local variable 1 and load that on tack again. We decided previously when we store some value in a local variable we use assignment to that variable and output that code. For clarity I added parentheses:

Output:CS_4_0001 = (a < b == 0)
Stack:CS_4_0001

-------------------------------------------------------------------------------------
IL_000a: brtrue.s IL_0019
--

We have got a conditional branch. We create a block starting from here to IL_0019. And put them in curly braces. And our condition is on the stack. We find a true condition branch so we negate it and put it as if structure as I said at the beginning.

Output:
if(!CS_4_0001)
{
IL_000c: nop
IL_000d: ldstr "Condition is true"
IL_0012: call void [mscorlib]System.Console::Write(string)
IL_0017: nop
IL_0018: nop
}

Stack: [Empty]

-------------------------------------------------------------------------------------
IL_0019: ldarg.2
IL_001a: stloc.0
IL_001b: br.s IL_001d
IL_001d: ldloc.0
IL_001e: ret

We do not parse them here. They are very simple to understand.
===========================================================

Ok we now can work on little more complex code3s than easiest. This will also produce codes that was generated by "for structure" but in a funny way. If we add a little more intelligence to produce "goto" output code for special branching that we cannot handle with if, we get following result.

The code like this:


for(int i=0;i<10;i++)
{
...
}
...

will be converted to:

int i;
i=0;
label_1:

if(i<10)
{
---
i++;
goto label_1;
}
...

For is not our today’s material. We'll look at loops next time.

=====================================================================================
Now you may find that our theory generates a funny code block like:

CS$4$0001=((a<b)==0);

if(!CS$4$0001)
{

---

}


Here the optimization comes to scene. But we skip them for future.

4 comments:

  1. Dear Maruf,
    I got MSIL code generated by Visual Studio 2005 compiler using reflection.

    Now I want to covert this MSIL code into C# code.

    How can I do it ? Can you provide an example ?

    Thanks in advance.

    Regards,
    Ibrahim Faisal

    ReplyDelete
  2. I am trying to build a tool. But there is already a lot of tools to convert MSIL to C#.

    Here is a link for a free tool:

    http://www.aisto.com/roeder/dotnet

    ReplyDelete
  3. Thanks.

    Actually I am trying to understand process of converting MSIL to C#.

    I think your tutorial may help.
    Thanks for reply.

    ReplyDelete
  4. Hi faisal,

    I'll try to continue this blog session. Within few months you may expect to get full details about the MSIL structure.

    I'll be very happy if you (to all readers also) add comments on the pages (criticisms are most welcome) and also inform me about the mistakes.

    Thanks

    ReplyDelete