Verify checksum: bad performance

[MarcinW]

2umbra:

For some reason the Delphi compiler (except some very simple cases, e.g. for Integer parameters) doesn't detect if the variable passed as an argument is being modified inside procedure/function. Even the newest Delphi XE2 behaves in the same way.

Using "const" is important even for non-reference-counted variables larger than 4 bytes - it prevents copying the arguments at the beginning of the procedure/function:

Code: Select all

type
  TTestRec = record
    X1 : Integer;
    X2 : Integer;
    X3 : Integer;
    X4 : Integer;
  end;

function Test1(Rec : TTestRec) : Integer;
begin
  with Rec do
    Result:=X1+X2+X3+X4;
end;

function Test2(const Rec : TTestRec) : Integer;
begin
  with Rec do
    Result:=X1+X2+X3+X4;
end;

Disassembled:

Code: Select all

Test2       proc near
            mov     edx, [eax]
            add     edx, [eax+4]
            add     edx, [eax+8]
            add     edx, [eax+0Ch]
            mov     eax, edx
            retn
Test2       endp

Code: Select all

Test1       proc near

var_RecCopy_X1  = dword ptr -18h
var_RecCopy_X2  = dword ptr -14h
var_RecCopy_X3  = dword ptr -10h
var_RecCopy_X4  = dword ptr -0Ch

            push    esi
            push    edi
            add     esp, 0FFFFFFF0h
            mov     esi, eax
            lea     edi, [esp+18h+var_RecCopy_X1]
            mov     ecx, 4
            rep movsd               ; COPYING
            mov     eax, [esp+18h+var_RecCopy_X1]
            add     eax, [esp+18h+var_RecCopy_X2]
            add     eax, [esp+18h+var_RecCopy_X3]
            add     eax, [esp+18h+var_RecCopy_X4]
            add     esp, 10h
            pop     edi
            pop     esi
            retn
Test1       endp