LLVM supports jcxz/jecxz, but doesn't allow overriding the register used by the 'loop' instruction.

// compile with:
//    clang -m32 test-loopw.c -O2 -o test-loopw
//
// expected output:
//    0xffff0000

#include <stdlib.h>
#include <stdio.h>

int main()
{
  unsigned x = 0xffff1234;
  // Intel syntax:
  //     asm volatile("1: addr16 loop 1b" : "+c" (x));
  asm volatile("1: loopw 1b" : "+c" (x));
  printf("%x\n", x);
  exit (x == 0xffff0000 ? 0 : 1);
}

This seems to affect the disassembler as well.  The above program is disassembled like this:

Disassembly of section .text:
main:
 ...
 8048354:	89 d9 	movl	%ebx, %ecx
 8048356:	67 e2 fd 	loop	-3
 ...