bug in longlong.h for aarch64 sub_ddmmss

[Vincent Lefevre]

On 2020-06-16 12:55:07 +0200, Vincent Lefevre wrote:
> In longlong.h from GMP 6.2.0:
> 
> #define sub_ddmmss(sh, sl, ah, al, bh, bl) \
>   do {                                                                  \
>     if (__builtin_constant_p (bl) && -(UDItype)(bl) < 0x1000)           \
>       __asm__ ("adds\t%1, %x4, %5\n\tsbc\t%0, %x2, %x3"                 \
>                : "=r,r" (sh), "=&r,&r" (sl)                             \
>                : "rZ,rZ" ((UDItype)(ah)), "rZ,rZ" ((UDItype)(bh)),      \
>                  "r,Z"   ((UDItype)(al)), "rI,r" (-(UDItype)(bl)) __CLOBBER_CC);\
>     else                                                                \
>       __asm__ ("subs\t%1, %x4, %5\n\tsbc\t%0, %x2, %x3"                 \
>                : "=r,r" (sh), "=&r,&r" (sl)                             \
>                : "rZ,rZ" ((UDItype)(ah)), "rZ,rZ" ((UDItype)(bh)),      \
>                  "r,Z"   ((UDItype)(al)), "rI,r"  ((UDItype)(bl)) __CLOBBER_CC);\
>   } while(0);
> 
> I don't understand this code. For instance, on ah=al=bh=bl=0, you
> do adds(0,0), which unsets the carry. And since borrow = not(carry),
> the sbc(0,0) will return -1.

Here's a test without using MPFR:

#define W_TYPE_SIZE 64
#define __GMP_DECLSPEC
#define __GMP_GNUC_PREREQ(maj, min) \
  ((__GNUC__ << 16) + __GNUC_MINOR__ >= ((maj) << 16) + (min))
#include <stdio.h>
#include "longlong.h"

int main (void)
{
  unsigned long sh, sl;

  sub_ddmmss (sh, sl, 0, 0, 0, 0);
  printf ("%lx\n", sh);
  return 0;
}

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Vincent Lefèvre <vincent at vinc17.net> - Web: <https://www.vinc17.net/>
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