delphi - local variable and array of TPair<Int,Int> - strange behavior of memory allocation

Question

i have following code example compiled in delphi xe5 update 2.

procedure TForm1.FormCreate(Sender: TObject);
var i,t:Integer;
    buf: array [0..20] of TPair<Integer,Integer>;
begin
  t := 0;
  for i := Low(buf) to High(buf) do begin
    ShowMessage(
      Format(
        'Pointer to i = %p;'#$d#$a+
        'Pointer to buf[%d].Key = %p;'#$d#$a+
        'Pointer to buf[%d].Value = %p;'#$d#$a+
        'Pointer to t = %p',
        [@i, i, @(buf[i].Key), i, @(buf[i].Value), @t]
      )
    );
    buf[i].Key := 0;
    buf[i].Value := 0;
    t := t + 1;
  end;
end;

if i run it it shows me the adresses of the variables. the variables i and t have adresses in the memory range of buf!
when ireaches 3 the assignment buf[i].Value := 0; overwrite first 3 bytes of i and the last byte of t. this results in a infinity loop because i gets allways reset to 0 when it reaches 3.
if i allocate the memory by myself with SetLength(buf,20); everything is fine.

the picture shows what i mean.

my setup:

• Windows 7 64 Bit
• Delphi XE 5 Update 2
• Debug Configuration 32 Bit

strange, isn't it?
can anybody reproduce it?
is it a bug in the delphi compiler?

thanks.

EDIT:
here is the same example, but maybe better to understand what i mean:

and btw.: sorry for my bad english ;)

Answer 1

This definitely looks like a compiler bug. It only affects an array of TPair allocated on the stack. For example, this compiles and runs fine :

program Project1;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  Generics.Collections;

var i:Integer;
    buf: array [0..20] of TPair<Integer,Integer>;
begin
  for i := Low(buf) to High(buf) do begin
    buf[i].Key := 0;
    buf[i].Value := 0;
  end;
end.

This, however, demonstrates the error :

program Project1;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  Generics.Collections;    

procedure DoSomething;
var i:Integer;
    buf: array [0..20] of TPair<Integer,Integer>;
begin
  for i := Low(buf) to High(buf) do begin
    buf[i].Key := 0;
    buf[i].Value := 0;
  end;
end;

begin
  DoSomething;
end.

The compiler seems to be miscalculating the size of a TPair<Integer,Integer>. The compiled assembly shows the preamble as follows :

Project1.dpr.14: begin
00445C50 55               push ebp
00445C51 8BEC             mov ebp,esp
00445C53 83C4E4           add esp,-$1c  //***  Allocate only 28 bytes (7words)
Project1.dpr.15: for i := Low(buf) to High(buf) do begin
00445C56 33C0             xor eax,eax
00445C58 8945FC           mov [ebp-$04],eax
Project1.dpr.16: buf[i].Key := 0;
00445C5B 8B45FC           mov eax,[ebp-$04]
00445C5E 33D2             xor edx,edx
00445C60 8954C5E7         mov [ebp+eax*8-$19],edx
Project1.dpr.17: buf[i].Value := 0;
00445C64 8B45FC           mov eax,[ebp-$04]
00445C67 33D2             xor edx,edx
00445C69 8954C5EB         mov [ebp+eax*8-$15],edx
Project1.dpr.18: end;
00445C6D FF45FC           inc dword ptr [ebp-$04]
Project1.dpr.15: for i := Low(buf) to High(buf) do begin
00445C70 837DFC15         cmp dword ptr [ebp-$04],$15
00445C74 75E5             jnz $00445c5b
Project1.dpr.19: end;
00445C76 8BE5             mov esp,ebp
00445C78 5D               pop ebp
00445C79 C3               ret 
00445C7A 8BC0             mov eax,eax

The compiler has only allocated 7 dwords on the stack. The first is for the integer i, leaving only 6 dwords allocated for the TPair array, which is insufficient (SizeOf(TPair<integer,integer>) is equal to 8 -> two dwords). On the third iteration, mov [ebp+eax*8-$15],edx (ie: buf[2].Value) runs into the stack location for i and sets its value to zero.

You can demonstrate a working program by forcing enough space on the stack :

program Project1;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  Generics.Collections;


procedure DoSomething;
var i:Integer;
    fixalloc : array[0..36] of Integer; // dummy variable
                                        // allocating enough space for
                                        // TPair array
    buf: array [0..20] of TPair<Integer,Integer>;
begin
  for i := Low(buf) to High(buf) do begin
    buf[i].Key := i;
    buf[i].Value := i;
  end;
end;

begin
  DoSomething;
end.

This is tested in XE2, but it seems this persists through at least XE5 if you are also seeing the problem.

Answer 2

It is clear that @J... is correct in identifying this as a compiler bug. From my tests I observe that it afflicts both 32 and 64 bit Windows versions of the compiler. I don't know about the OSX compiler, or the mobile compilers.

There are some reasonable workarounds available. This problem produces reasonable output:

{$APPTYPE CONSOLE}
uses
  System.SysUtils, Generics.Collections;

type
  TFixedLengthPairArray = array [0..20] of TPair<Integer,Integer>;

procedure DoSomething;
var
  i: Integer;
  buf: TFixedLengthPairArray;
begin
  Writeln(Format('%p %p', [@i, @buf]));
end;

begin
  DoSomething;
end.

Likewise this one:

{$APPTYPE CONSOLE}
uses
  System.SysUtils, Generics.Collections;

type
  TFixedLengthPairArray = array [0..20] of TPair<Integer,Integer>;

procedure DoSomething;
var
  i: Integer;
  buf: array [0..20] of TPair<Integer,Integer>;
begin
  Writeln(Format('%p %p', [@i, @buf]));
end;

begin
  DoSomething;
end.

Or indeed this:

{$APPTYPE CONSOLE}
uses
  System.SysUtils, Generics.Collections;

type
  TPairOfIntegers = TPair<Integer,Integer>;

procedure DoSomething;
var
  i: Integer;
  buf: array [0..20] of TPairOfIntegers;
begin
  Writeln(Format('%p %p', [@i, @buf]));
end;

begin
  DoSomething;
end.

And even this:

{$APPTYPE CONSOLE}
uses
  System.SysUtils, Generics.Collections;

type
  TPairOfIntegers = TPair<Integer,Integer>;

procedure DoSomething;
var
  i: Integer;
  buf: array [0..20] of TPair<Integer,Integer>;
begin
  Writeln(Format('%p %p', [@i, @buf]));
end;

begin
  DoSomething;
end.

And this:

{$APPTYPE CONSOLE}
uses
  System.SysUtils, Generics.Collections;

procedure DoSomething;
type
  TPairOfIntegers = TPair<Integer,Integer>;
var
  i: Integer;
  buf: array [0..20] of TPair<Integer,Integer>;
begin
  Writeln(Format('%p %p', [@i, @buf]));
end;

begin
  DoSomething;
end.

---

So it seems that so long as the compiler has already instantiated the generic type before it encounters the local variable declaration, it is able to reserve the correct stack size.