Q Why does the dec/ jne combo operate faster than the equivalent loopnz? A The dec/ jnz pair operates faster then a loopsz for several reasons.What follows are a number of basic ways to compact shellcodes. PUBLIC EuclidsGCD TEXT SEGMENT m$ = 8 size = 4 n$ = 12 size = 4 EuclidsGCD PROC NEAR Line 7 mov eax, DWORD PTR m$ esp-4 push esi mov esi, DWORD PTR n$ esp cdq idiv esi mov ecx, edx Line 8 test ecx, ecx je SHORT $L563 $L547: Line 12 mov eax, esi cdq idiv ecx Line 13 mov esi, ecx mov ecx, edx test ecx, ecx jne SHORT $L547 $L563: Line 10 mov eax, esi pop esi Line 15 ret 0 EuclidsGCD ENDP TEXT ENDS ENDAltering the conditions to maintain the same structure gives us. The instruction 'mov ecx, edx' moves r into ecx, while q is not used for the rest of the loop, and is therefore discarded.Example: Decompile Optimized Code. When the instruction 'idiv esi' is executed, eax contains the quotient (q), and edx contains the remainder (r).

PUBLIC EuclidsGCD TEXT SEGMENT m$ = 8 size = 4 n$ = 12 size = 4 EuclidsGCD PROC NEAR Line 7 mov eax, DWORD PTR m$ esp-4 push esi mov esi, DWORD PTR n$ esp cdq idiv esi mov ecx, edx Line 8 test ecx, ecx je SHORT $L563 $L547: Line 12 mov eax, esi cdq idiv ecx Line 13 mov esi, ecx mov ecx, edx test ecx, ecx jne SHORT $L547 $L563: Line 10 mov eax, esi pop esi Line 15 ret 0 EuclidsGCD ENDP TEXT ENDS ENDAt the beginning of the function, eax contains m, and esi contains n. Another fact is, that the C code for an infinite FOR loop.

Because this is an infinite loop, we can assume that these lines are unnecessary.' Mov eax, 1' initializes eax.the test immediately afterwards tests the value of eax to ensure that it is nonzero.because eax will always be nonzero (eax = 1) at this point, the conditional jump can be removed along whith the 'mov' and the 'test'.The assembly is actually checking whether 1 equals 1. Mov eax, 1 test eax, eax je SHORT $L473The code in this line is the code generated for the 'while( 1 )' C code, to be exact, it represents the loop break condition. The compiler is very liberal with how it assigns storage in the function, and readily discards values that are not needed.Example: Manual Optimization. Also, the value of r is tested twice, in the same places. The structure of the function is altered greatly: the division of m and n to produce q and r is performed in this function twice: once at the beginning of the function to initialize, and once at the end of the loop.

Test eax eax