如何在内联汇编中实现这一点？

Question

I'm woefully bad at understanding the GNU inline assembly syntax, so I'm hoping a practical example may help. Given the following assembly (x86-64, output by Clang) how would I construct a function using inline assembly that would be identical? GCC produces different code for the same function and I would like to get it to produce an identical version to what Clang (-O3) outputs.

bittest(unsigned char, int):
    btl %esi, %edi
    setb    %al
    ret

Here is what GCC (-O3) is producing:

bittest(unsigned char, int):
    movzx    eax, dil
    mov    ecx, esi
    sar    eax, cl
    and    eax, 1
    ret

Here is the C code for the function:

bool bittest(unsigned char byte, int index)
{
    return (byte >> index) & 1;
}

Answer 1

Well, last time I wrote a 32bit bittest, it looked something like this (the 64bit looks slightly different):

unsigned char _bittest(const long *Base, long Offset) 
{ 
   unsigned char old; 
   __asm__ ("btl %[Offset],%[Base] ; setc %[old]" : 
      [old] "=rm" (old) : 
      [Offset] "Ir" (Offset), [Base] "rm" (*Base) : 
      "cc"); 

   return old; 
}

Although if you want to put it in a public header, I have a different version. When I use -O2, it ends up inlining the whole thing to make really efficient code.

I'm surprised gcc doesn't generate the btl here itself (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36473 ), but you are right it doesn't.

Answer 2

I think it's unlikely that you can nail down a byte-by-byte equivalent version in your compiler, there are minor differences that aren't worth worrying about. Following this question , make sure you're compiling with the correct flags. Trying to get two compilers to produce identical output is probably an exercise in futility.

Answer 3

If you want to generate the exact same code then you can do the following

const char bittestfunction[] = { 0xf, 0xa3, 0xf7, 0xf, 0x92, 0xc0, 0x3 };
int (*bittest)( unsigned char, int ) = (int(*)(unsigned char, int))bittestfunction;

You can call this in the same way bittest( foo, bar ) .

From objdump on the (gcc) compiled executable

00000000004006cc <bittestfunction>:
  4006cc:       0f a3 f7                bt     %esi,%edi
  4006cf:       0f 92 c0                setb   %al
  4006d2:       c3                      retq