faster implementation of sum ( for Codility test )

Question

How can the following simple implementation of sum be faster?

private long sum( int [] a, int begin, int end ) {
    if( a == null   ) {
        ret         


        

Answer 1

{In Pascal + Assembly}
{$ASMMODE INTEL}
function equi (A : Array of longint; n : longint ) : longint;
var c:Longint;
    label noOverflow1;
    label noOverflow2;
    label ciclo;
    label fine;
    label Over;
    label tot;
Begin
 Asm
    DEC n
    JS over
    XOR ECX, ECX   {Somma1}
    XOR EDI, EDI   {Somma2}
    XOR EAX, EAX
    MOV c, EDI
    MOV ESI, n
  tot:
    MOV EDX, A
    MOV EDX, [EDX+ESI*4]
    PUSH EDX
    ADD ECX, EDX
    JNO nooverflow1
    ADD c, ECX
    nooverflow1:
    DEC ESI
  JNS tot;
    SUB ECX, c
    SUB EDI, c
  ciclo:
    POP EDX
    SUB ECX, EDX
    CMP ECX, EDI
    JE fine
    ADD EDI, EDX
    JNO nooverflow2
    DEC EDI
    nooverflow2:
    CMP EAX, n
    JA over
    INC EAX
    JMP ciclo
    over:
      MOV EAX, -1
    fine:
  end;
End;