CLRHack: FibBenchmark

The first thing to look at is the Fibonacci benchmark. The source code is here:

(in-package "CLRHACK")

(progn
  (defun fib (n)
    (if (< n 2)
        n
        (+ (fib (- n 1))
           (fib (- n 2)))))
  (defun main ()
    (print "Fibonacci of 10:")
    (print (fib 10)))
  (main))

And it compiles to this IL code: (commentary after the code)

.assembly extern mscorlib {}
.assembly extern LispBase {}

.assembly 'FibBenchmark' {}
.module 'FibBenchmark.exe'

.class public auto ansi beforefieldinit Program
       extends [mscorlib]System.Object
{
    .field public static class [LispBase]Lisp.Symbol 'SYM_G545'

.method public static hidebysig specialname rtspecialname void '.cctor'() cil managed
{
    .maxstack 8
    ldsfld class [LispBase]Lisp.Package [LispBase]Lisp.Package::CommonLisp
    ldstr "T"
    callvirt instance class [LispBase]Lisp.Symbol [LispBase]Lisp.Package::'Intern'(string)
    stsfld class [LispBase]Lisp.Symbol Program::'SYM_G545'
    ret
}

.method public static hidebysig object 'FIB'(object) cil managed
{
    .maxstack 8
    .locals (object TEMP_B)
    ldarg 0
    ldc.i4 2
    box int32
    stloc TEMP_B
    unbox.any int32
    ldloc TEMP_B
    unbox.any int32
    clt
    brtrue TRUE543
    ldnull
    br END544
TRUE543:
    nop
    ldsfld class [LispBase]Lisp.Symbol Program::'SYM_G545'
END544:
    nop
    ldnull
    ceq
    brtrue ELSE546
    ldarg 0
    ret
ELSE546:
    nop
    ldarg 0
    ldc.i4 1
    box int32
    stloc TEMP_B
    unbox.any int32
    ldloc TEMP_B
    unbox.any int32
    sub
    box int32
    call object Program::'FIB'(object)
    ldarg 0
    ldc.i4 2
    box int32
    stloc TEMP_B
    unbox.any int32
    ldloc TEMP_B
    unbox.any int32
    sub
    box int32
    call object Program::'FIB'(object)
    stloc TEMP_B
    unbox.any int32
    ldloc TEMP_B
    unbox.any int32
    add
    box int32
    ret
BLOCK_END_FIB_532:
    nop
    ret
}

.method public static hidebysig object 'MAIN'() cil managed
{
    .maxstack 8
    ldstr "Fibonacci of 10:"
    call void [mscorlib]System.Console::'WriteLine'(object)
    ldnull
    pop
    ldc.i4 10
    box int32
    call object Program::'FIB'(object)
    call void [mscorlib]System.Console::'WriteLine'(object)
    ldnull
    ret
BLOCK_END_MAIN_536:
    nop
    ret
}

.method public static hidebysig void 'Main'() cil managed
{
    .entrypoint
    .maxstack 8
    call object Program::'MAIN'()
    pop
    ret
}

} // end of class Program

The first thing the fib program does is compare argument x to the literal number 2. The compiler pushes argument 0 on to the stack, and then the compiler pushes a integer 2 on to the stack and boxes it.

Next, the compiler has to perform the compare. In order to do this it must unbox both arguments. One argument is on top of the stack, so it is put into a local TEMP_B so we can get to the other argument. We unbox it. We then restore TEMP_B to the top of stack and unbox it. Finally we compare the two unboxed values for less than.

This pattern of unboxing a pair of elements from the top of stack by way of a temporary local is repeated several places in the compiled code as FIB rather inefficiently subtracts 1 or 2 from the argument and makes the recursive call.

This example shows that the compiler basically treats everything as a .NET object. It unboxes numbers at the last moment and boxes the results as soon as they are generated. It is not efficient code.