Thread: ASM function

Hello,

my task is to create following function:

Code:

unsigned char nthbyte_x(unsigned char n, unsigned long x) {
   __asm {
      ...
   }
}

Its result should be the value of nth byte in number x. Could you give me some basic explanation of how should I do that, or some commands, I should use? Thank you.

I like to deal with asm, as any optimization junkie does, but keep in mind that programming directly in asm will restrict your code to a specific family of processors.

For example, the Salem's code, for x86 in i686 mode will compile as:

Code:

nthbyte_x:
  movzx ecx, BYTE [esp+4]
  mov eax, [esp+8]
  sal ecx, 3
  shr eax, cl
  ret

But on x86 in amd64 mode, it will be:

Code:

nthbyte_x:
  movzx edi, dil
  mov rax, rsi
  lea ecx, [rdi*8]
  shr rax, cl
  ret

And on ARM (AArch32, for Cortex-A53):

Code:

nthbyte_x:
  lsl r0, r0, #3
  lsr r0, r1, r0
  uxtb  r0, r0
  bx  lr

But on ARM (AArch64, for Cortex-A53):

Code:

nthbyte_x:
  ubfiz w0, w0, 3, 8
  lsr x0, x1, x0
  ret

And, to cite GCC example, there are other 55 (see GCC documetation) "kinds" of assembly.

So, using asm your code is not portable... If you want to restrict your implementation to one of them, it is ok to use asm, if not DON'T USE IT!