Const parameters are an implementation detail

A parameter with a const is different from one without const in terms of a function signature. But is that actually true? Is A(x: T) really different from B(const x: T)?

Disclaimer: In this article I am focusing on the default calling convention on Windows in Delphi which is register.

We will go onto a journey through some assembler code and look into various method calls with different typed parameters both const and no const and explore their differences. Don’t worry - I will explain the assembler in detail.

I have following code for you that we will use with different types to explore:

{$APPTYPE CONSOLE}
{$O+,W-}
type
  TTestType = …;

procedure C(z: TTestType);
begin
end;

procedure A(y: TTestType);
begin
  C(y);
end;

procedure B(const y: TTestType);
begin
  C(y);
end;

procedure Main;
var
  x: TTestType;
begin
  A(x);
  B(x);
end;

We will put different types for TTestType, put a breakpoint into Main and see how the code looks like for the calls and inside of A and B. I will be using 10.4.2 for this experiment. We will ignore the warning because it does not matter for the experiment.

Let’s start with something simple: Integer

When we look into the disassembly we see the following:

Hamlet.dpr.24: A(x);
00408FA9 8BC3             mov eax,ebx
00408FAB E8E8FFFFFF       call A
Hamlet.dpr.25: B(x);
00408FB0 8BC3             mov eax,ebx
00408FB2 E8E9FFFFFF       call B

eax is the register being used for the first and only parameter we have in our routines. ebx is the register where x is being stored during the execution of Main. No difference between the two calls though – and here is how it looks inside of A and B. In case you wonder – the calls to C are just there to prevent the compiler to optimize away some code which will be important later.

Hamlet.dpr.12: C(y);
00408F98 E8F7FFFFFF       call C
Hamlet.dpr.13: end;
00408F9D C3               ret 
Hamlet.dpr.17: C(y);
00408FA0 E8EFFFFFFF       call C
Hamlet.dpr.18: end;
00408FA5 C3               ret

Also the same, the value is being kept in the register it was being passed in – eax – and C is called. We get the same identical code for various other ordinal types such as Byte, SmallInt or Cardinal, enums or sets that are up to 4 byte in size, Pointer or TObject (did someone say “not on ARC”? Well we are on 10.4.2 – I erased that from my memory already – and soon from my code as well).

Let’s explore Int64 next:

Hamlet.dpr.24: A(x);
00408FC7 FF742404         push dword ptr [esp+$04]
00408FCB FF742404         push dword ptr [esp+$04]
00408FCF E8C8FFFFFF       call A
Hamlet.dpr.25: B(x);
00408FD4 FF742404         push dword ptr [esp+$04]
00408FD8 FF742404         push dword ptr [esp+$04]
00408FDC E8CFFFFFFF       call B

And B and C:

Hamlet.dpr.11: begin
00408F9C 55               push ebp
00408F9D 8BEC             mov ebp,esp
Hamlet.dpr.12: C(y);
00408F9F FF750C           push dword ptr [ebp+$0c]
00408FA2 FF7508           push dword ptr [ebp+$08]
00408FA5 E8EAFFFFFF       call C
Hamlet.dpr.13: end;
00408FAA 5D               pop ebp
00408FAB C20800           ret $0008

Hamlet.dpr.16: begin
00408FB0 55               push ebp
00408FB1 8BEC             mov ebp,esp
Hamlet.dpr.17: C(y);
00408FB3 FF750C           push dword ptr [ebp+$0c]
00408FB6 FF7508           push dword ptr [ebp+$08]
00408FB9 E8D6FFFFFF       call C
Hamlet.dpr.18: end;
00408FBE 5D               pop ebp
00408FBF C20800           ret $0008

On 32bit an Int64 is passed on the stack as you can see – nothing fancy here but reserving a stack frame and passing on the value to C. But no difference between the two routines.

I don’t want to bore you death with walls of identical assembler code so I can tell you that for all the floating point types, enums and sets regardless their size the calls to A and B are always identical.

But now for the first difference – lets take a look at set of Byte – the biggest possible set you can have in Delphi. A set is actually a bitmask with one bit for every possible value – in this case this type is 256 bit in size or 32 byte. Like many other types that don’t fit into a register it will actually be passed by reference – this time I also show you the prologue of Main where you can see that the compiler reserved 32 byte on the stack for our x variable and that this time the address of the value is being passed to A and B which is in the esp register - the stack pointer:

Hamlet.dpr.23: begin
00408FC8 83C4E0           add esp,-$20
Hamlet.dpr.24: A(x);
00408FCB 8BC4             mov eax,esp
00408FCD E8C6FFFFFF       call A
Hamlet.dpr.25: B(x);
00408FD2 8BC4             mov eax,esp
00408FD4 E8DFFFFFFF       call B

Let’s take a look into A where we can see the first difference:

Hamlet.dpr.11: begin
00408F98 56               push esi
00408F99 57               push edi
00408F9A 83C4E0           add esp,-$20
00408F9D 8BF0             mov esi,eax
00408F9F 8D3C24           lea edi,[esp]
00408FA2 B908000000       mov ecx,$00000008
00408FA7 F3A5             rep movsd 
Hamlet.dpr.12: C(y);
00408FA9 8BC4             mov eax,esp
00408FAB E8E4FFFFFF       call C
Hamlet.dpr.13: end;
00408FB0 83C420           add esp,$20
00408FB3 5F               pop edi
00408FB4 5E               pop esi
00408FB5 C3               ret

First the non-volatile registers (that means their values need to be preserved over subroutine calls, in other words, when A returns the values in those registers must be the same as they were before A was being called) are being saved. Then we see again reserving 32byte on the stack as previously.

The next four instructions: the parameter being passed eax is stored in esi, that is the address to our value x that we passed in y, then the address of esp is loaded into edi. The rep instruction does something interesting: it repeats the movsd instruction (not to be confused with the movsd – hey it’s called complex instruction set for a reason) –moving 4 byte as often as ecx says using esi as source address and edi as destination. Long story short, it does a local copy of our set.

There we have our first difference – the compiler produces a copy of our parameter value – if the C call would not be here the compiler would actually optimize that away as it detects there is nothing to do with y. And it does so even though we just read the value and not actually modify it as we could with a non const parameter.

Finally the type many of you have been waiting for: string

Hamlet.dpr.24: A(x);
004F088B 8B45FC           mov eax,[ebp-$04]
004F088E E88DFFFFFF       call A
Hamlet.dpr.25: B(x);
004F0893 8B45FC           mov eax,[ebp-$04]
004F0896 E8CDFFFFFF       call B

Since string is a managed type – meaning the compiler inserts code that ensures its initialized as empty string and gets finalized we got quite some code in Main this time and the variable is on the stack – string is a reference type so in terms of size its just the same as a pointer fitting in eax. Just to avoid confusion – string is a reference type but the parameter is by value – the value of the string reference is passed in the eax register just like earlier for Integer and alike.

Fasten your seatbelts as we take a look into the code of A:

Hamlet.dpr.11: begin
004F0820 55               push ebp
004F0821 8BEC             mov ebp,esp
004F0823 51               push ecx
004F0824 8945FC           mov [ebp-$04],eax
004F0827 8B45FC           mov eax,[ebp-$04]
004F082A E8D994F1FF       call @UStrAddRef
004F082F 33C0             xor eax,eax
004F0831 55               push ebp
004F0832 685D084F00       push $004f085d
004F0837 64FF30           push dword ptr fs:[eax]
004F083A 648920           mov fs:[eax],esp
Hamlet.dpr.12: C(y);
004F083D 8B45FC           mov eax,[ebp-$04]
004F0840 E8AFFFFFFF       call C
Hamlet.dpr.13: end;
004F0845 33C0             xor eax,eax
004F0847 5A               pop edx
004F0848 59               pop ecx
004F0849 59               pop ecx
004F084A 648910           mov fs:[eax],edx
004F084D 6864084F00       push $004f0864
004F0852 8D45FC           lea eax,[ebp-$04]
004F0855 E8CA93F1FF       call @UStrClr
004F085A 58               pop eax
004F085B FFE0             jmp eax
004F085D E9DA89F1FF       jmp @HandleFinally
004F0862 EBEE             jmp $004f0852
004F0864 59               pop ecx
004F0865 5D               pop ebp
004F0866 C3               ret

Ok, that’s quite a lot of code but the important thing is this - the compiler basically turned the code into this:

var
  z: Pointer;
begin
  z := Pointer(y);
  System._UStrAddRef(z);
  try
    C(z);
  finally
    System._UStrClr(z);
  end;
end;

The System routine _UStrAddRef checks if the string is not empty and increases its reference count if it’s not a reference to a constant string because they have a reference count of -1 meaning they are not allocated on the heap but point to some location in your binary where the string constant is located. _UStrClr clears the passed string variable and reduces its reference count under the same conditions.

So why is that being done? The string was passed as non const causing the compiler to ensure that nothing goes wrong with the string by bumping its reference count for the time of the execution of this routine. If some code would actually modify the string the code to do that would see that the string – assuming we have one that had a reference count of at least 1 when entering the method – has a reference count of at least 2. This would trigger the copy on write mechanic and keep the string that was originally passed to the routine intact because even though a string is a reference type underneath it also has characteristics of a value type meaning that when we modify it we don’t affect anyone else that also holds a reference.

Now the code of B:

Hamlet.dpr.17: C(y);
004F0868 E887FFFFFF       call C
Hamlet.dpr.18: end;
004F086D C3               ret

That’s almost disappointing after so much code in A. But joking aside the const had a significant effect: all that reference bumping, try finally to ensure proper reference dropping in case of an exception is not necessary because the parameter being const ensures that it cannot be modified.

The code being executed in A is not the end of the world or will utterly destroy the performance of your application but is completely unnecessary most of the time. And in many cases it can indeed mean the difference for the fast execution of an algorithm. Yes, I know we can construct some terrible code that causes bugs when the parameter is const because we have a circular data dependency but let’s not talk about this – because this has already been discussed.

Similar code can be found for interfaces, dynamic arrays and managed records (both kinds, records with managed field types and the new custom managed records).

There are not many more types left – records and static arrays depending on their size are either passed by value or reference or in the fancy case of being of size 3 pushed onto the stack.

For a complete summary please refer to this table.

As we can see from the caller side a const and a non const parameter are not any different for the register calling convention. For other calling conventions however they differ – especially on non Windows platforms. As an example with the stdcall convention a record that is bigger than a pointer is being pushed onto the stack as a non const parameter while as a const parameter just its address is being pushed so basically passed by reference.

That was a rather lengthy adventure – but what did we learn?

For most code on Windows a const parameter is actually an implementation detail and does not affect the code on the caller side. But knowing that it depends on the platform and the calling convention makes it clear that as much as some would like non const and const parameters to be compatible in order to easily add some missing const throughout the RTL or VCL where it would have a benefit of removing unnecessary instructions is simply impossible. Nor could we simply assume non const parameters to behave similar to const parameters.

Using const parameters can avoid unnecessary instructions and speed up your code without any cost or side effects. There is no reason to not make your string, interface, dynamic array or record parameters const. That can be even more important if you are a library developer.

In the next article we will explore another kind of parameter: out