Function return parameter code generation misery
lazarus / 67-Windows

----------------------------------------------------------------------
cryborg2 · 2022-11-05 16:09

How to tell the compiler to generate a code that treat Pointer type
function return parameter as a "var" parameter just like it does with
string type function return parameters?


function Test1{EAX}: string; assembler;
asm
// from here it is like we have a header like this: procedure
Test1(var S{EAX}: string); assembler;
end;

function Test2{EAX}: Pointer; assembler;
asm
// from here it is like we have a header like this: procedure
Test2(out P{EAX}: Pointer); assembler;
end;


procedure Test;
  var
    s: string;
    P: Pointer;
begin
  GetMem(P, MemBytes);

  s:=Test1; // In this case we pass the string to Test1 as a "var"
parameter! Then Test1 can access the string as it was a real "var"
parameter.
  {
    assembler:
    LEA     EAX,DWORD PTR [EBP-12]      (lea -0xc(%ebp),%eax)
    CALL    <TEST1>
  }

  P:=Test2; // In this case we not pass the pointer's address, we just
get an address defined by Test2.
  {
    assembler:
    CALL    <TEST2>
    MOV     DWORD PTR [EBP-16],EAX      (mov %eax,-0x10(%ebp))
  }

  FreeMem(P);
end;

----------------------------------------------------------------------
cryborg2 · 2022-11-05 18:45

This is important to me because I want to use my custom string data
type with the same fashion as the built-in strings (except for
concatenation of course).

----------------------------------------------------------------------
PascalDragon · 2022-11-05 20:36

How types are passed is fixed by the ABI and is not up to a user. So a
Pointer will always be passed by value in the result register.

----------------------------------------------------------------------
alpine · 2022-11-05 22:44

Quote from: cryborg2 on November 05, 2022, 07:45:29 pmThis is
important to me because I want to use my custom string data type with
the same fashion as the built-in strings (except for concatenation of
course).
IMO It is due to the fact that String have a special treatment in FPC.
With the help of advanced records and operator/function overloading
you can define whatever custom data type you want.
What do you mean with "the same fashion as"?

----------------------------------------------------------------------
cryborg2 · 2022-11-06 02:14

Quote from: y.ivanov on November 05, 2022, 11:44:37 pmQuote from:
cryborg2 on November 05, 2022, 07:45:29 pmThis is important to me
because I want to use my custom string data type with the same fashion
as the built-in strings (except for concatenation of course).
IMO It is due to the fact that String have a special treatment in FPC.
With the help of advanced records and operator/function overloading
you can define whatever custom data type you want.
What do you mean with "the same fashion as"?

I want to use the ":=" operator to get my virtual string type return
value, and to do so, the called function must have acces to my
VirtualString object -which have it's own memory management- to
transfer data through function return mechanism.

----------------------------------------------------------------------
PascalDragon · 2022-11-06 15:06

Then you need to use advanced records with operator overloads.
Anything else is just hacks.

----------------------------------------------------------------------
ASerge · 2022-11-06 17:01

Quote from: cryborg2 on November 06, 2022, 03:14:28 amI want to use
the ":=" operator to get my virtual string type return value, and to
do so, the called function must have acces to my VirtualString object
-which have it's own memory management- to transfer data through
function return mechanism.
You can, but not Pointer. Example:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i {$MODE
OBJFPC}{$LONGSTRINGS ON}{$MODESWITCH ADVANCEDRECORDS}{$APPTYPE
CONSOLE}type  TVirtualString = record  strict private    FLength:
SizeInt;    FData: PAnsiChar;    procedure CopyFrom(const Src;
ALength: SizeInt);    class operator Copy(constref Src:
TVirtualString; var Dst: TVirtualString);    class operator
Finalize(var Self: TVirtualString);    class operator Initialize(var
Self: TVirtualString);  public    class operator := (const S: string):
TVirtualString;    function AsString: string;  end; class operator
TVirtualString. := (const S: string): TVirtualString;begin 
Result.CopyFrom(Pointer(S)^, Length(S));end; function
TVirtualString.AsString: string;begin  SetString(Result, FData,
FLength);end; class operator TVirtualString.Copy(constref Src:
TVirtualString; var Dst: TVirtualString);begin 
Dst.CopyFrom(Src.FData^, Src.FLength);end; class operator
TVirtualString.Finalize(var Self: TVirtualString);begin 
ReAllocMem(Self.FData, 0);  Self.FLength := 0;end; class operator
TVirtualString.Initialize(var Self: TVirtualString);begin 
Self.FLength := 0;  Self.FData := nil;end; procedure
TVirtualString.CopyFrom(const Src; ALength: SizeInt);begin  FLength :=
ALength;  ReAllocMem(FData, FLength);  if FLength > 0 then   
Move(Src, FData^, FLength)end; // End TVirtualString function
SomeFunction: TVirtualString;begin  Result := '123';end; var  V:
TVirtualString;begin  V := '456';  V := SomeFunction; // As you want 
Writeln(V.AsString);  Readln;end.