Reverse Engineering challenge #10.

Tags: MIPS L1 X64 ASM ARM64 ARM .

This code snippet is short, but tricky. What does it do? It's used heavily in low-level programming and is well-known to many low-level programmers. There are several ways to calculate it, and this is the one of them.

Optimizing GCC 4.8.2:


f:
        lea     eax, [rdi-1+rsi]
        neg     esi
        and     eax, esi
        ret

Optimizing GCC 4.9.3 for ARM64:


f:
        add     w0, w0, w1
        neg     w1, w1
        sub     w0, w0, #1
        and     w0, w0, w1
        ret

(ARM) Optimizing Keil 5.05 (Thumb mode):


f PROC
        ADDS     r0,r0,r1
        SUBS     r0,r0,#1
        SUBS     r1,r1,#1
        BICS     r0,r0,r1
        BX       lr
        ENDP

Optimizing Keil 5.05 for ARM mode generates nearly the same code, so it's omitted here.

Optimizing GCC 4.4.5 for MIPS:


f:
        addiu   $4,$4,-1
        addu    $4,$4,$5
        subu    $2,$0,$5
        j       $31
        and     $2,$2,$4

More challenges: challenges.re; about solutions: challenges.re/#Solutions.