count_leading_zeros on 64-bit ppc

[Torbjorn Granlund]

David Harvey <dmharvey at math.harvard.edu> writes:

  I think there may be a bug in count_leading_zeros in longlong.h on 64- 
  bit PPC platforms. This is GMP 4.2.1.
  
  This is a bit worrying since I looked in longlong.h, and it looks  
  like assembly is used directly:
  
  #define count_leading_zeros(count, x) \
     __asm__ ("cntlzd %0,%1" : "=r" (count) : "r" (x))
  
  So if I'm reading this right, it might be a bug in the chip itself.
  
  
  Here is an example program that illustrates the bug:
  
  ======================================
  
  #define UWtype unsigned long
  #define UHWtype unsigned long
  #define UDWtype unsigned long
  #define W_TYPE_SIZE 64
  
  #include <stdio.h>
  #include <gmp.h>
  #include "longlong.h"
  
  int main()
  {
      unsigned long x = 0x0000000001379102;
      unsigned long c;
  
      count_leading_zeros(c, x);
      printf("%d\n", c);
  
      return 0;
  }
  
  ======================================
  
You're not using the macro properly.
Your example works wrong, perhaps because it gets a 32-bit gmp.h,
or something else.

But in a proper 64-bit gmp context, it works right.

I'll let you debug the exact cause of the failure.  :-)  

-- 
Torbjörn