Out of curiosity I was trying to generate a tail call opcode using C#. Fibinacci is an easy one, so my c# example looks like this:
private static void Main(string[] args)
{
Console.WriteLine(Fib(int.MaxValue, 0));
}
public static int Fib(int i, int acc)
{
if (i == 0)
{
return acc;
}
return Fib(i - 1, acc + i);
}
If I build it in release and run it without debugging I do not get a stack overflow. Debugging or running it without optimizations and I do get a stack overflow, implying that tail call is working when in release with optimizations on (which is what I expected).
The MSIL for this looks like this:
.method public hidebysig static int32 Fib(int32 i, int32 acc) cil managed
{
// Method Start RVA 0x205e
// Code Size 17 (0x11)
.maxstack 8
L_0000: ldarg.0
L_0001: brtrue.s L_0005
L_0003: ldarg.1
L_0004: ret
L_0005: ldarg.0
L_0006: ldc.i4.1
L_0007: sub
L_0008: ldarg.1
L_0009: ldarg.0
L_000a: add
L_000b: call int32 [ConsoleApplication2]ConsoleApplication2.Program::Fib(int32,int32)
L_0010: ret
}
I would've expected to see a tail opcode, per the msdn, but it's not there. This got me wondering if the JIT compiler was responsible for putting it in there? I tried to ngen the assembly (using ngen install <exe>
, navigating to the windows assemblies list to get it) and load it back up in ILSpy but it looks the same to me:
.method public hidebysig static int32 Fib(int32 i, int32 acc) cil managed
{
// Method Start RVA 0x3bfe
// Code Size 17 (0x11)
.maxstack 8
L_0000: ldarg.0
L_0001: brtrue.s L_0005
L_0003: ldarg.1
L_0004: ret
L_0005: ldarg.0
L_0006: ldc.i4.1
L_0007: sub
L_0008: ldarg.1
L_0009: ldarg.0
L_000a: add
L_000b: call int32 [ConsoleApplication2]ConsoleApplication2.Program::Fib(int32,int32)
L_0010: ret
}
I still don't see it.
I know F# handles tail call well, so I wanted to compare what F# did with what C# did. My F# example looks like this:
let rec fibb i acc =
if i = 0 then
acc
else
fibb (i-1) (acc + i)
Console.WriteLine (fibb 3 0)
And the generated IL for the fib method looks like this:
.method public static int32 fibb(int32 i, int32 acc) cil managed
{
// Method Start RVA 0x2068
// Code Size 18 (0x12)
.custom instance void [FSharp.Core]Microsoft.FSharp.Core.CompilationArgumentCountsAttribute::.ctor(int32[]) = { int32[](Mono.Cecil.CustomAttributeArgument[]) }
.maxstack 5
L_0000: nop
L_0001: ldarg.0
L_0002: brtrue.s L_0006
L_0004: ldarg.1
L_0005: ret
L_0006: ldarg.0
L_0007: ldc.i4.1
L_0008: sub
L_0009: ldarg.1
L_000a: ldarg.0
L_000b: add
L_000c: starg.s acc
L_000e: starg.s i
L_0010: br.s L_0000
}
Which, according to ILSpy, is equivalent to this:
[Microsoft.FSharp.Core.CompilationArgumentCounts(Mono.Cecil.CustomAttributeArgument[])]
public static int32 fibb(int32 i, int32 acc)
{
label1:
if !(((i != 0)))
{
return acc;
}
(i - 1);
i = acc = (acc + i);;
goto label1;
}
So F# generated tail call using goto statements? This isn't what I was expecting.
I'm not trying to rely on tail call anywhere, but I am just curious where exactly does that opcode get set? How is C# doing this?
C# compiler does not give you any guarantees about tail-call optimizations because C# programs usually use loops and so they do not rely on the tail-call optimizations. So, in C#, this is simply a JIT optimization that may or may not happen (and you cannot rely on it).
F# compiler is designed to handle functional code that uses recursion and so it does give you certain guarantees about tail-calls. This is done in two ways:
if you write a recursive function that calls itself (like your fib
) the compiler turns it into a function that uses loop in the body (this is a simple optimization and the produced code is faster than using a tail-call)
if you use a recursive call in a more complex position (when using continuation passing style where function is passed as an argument), then the compiler generates a tail-call instruction that tells the JIT that it must use a tail call.
As an example of the second case, compile the following simple F# function (F# does not do this in Debug mode to simplify debugging, so you may need Release mode or add --tailcalls+
):
let foo a cont = cont (a + 1)
The function simply calls the function cont
with the first argument incremented by one. In continuation passing style, you have a long sequence of such calls, so the optimization is crucial (you simply cannot use this style without some handling of tail calls). The generates IL code looks like this:
IL_0000: ldarg.1
IL_0001: ldarg.0
IL_0002: ldc.i4.1
IL_0003: add
IL_0004: tail. // Here is the 'tail' opcode!
IL_0006: callvirt instance !1
class [FSharp.Core] Microsoft.FSharp.Core.FSharpFunc`2<int32, !!a>::Invoke(!0)
IL_000b: ret