bug in longlong.h for aarch64 sub_ddmmss
Vincent Lefevre
vincent at vinc17.net
Tue Jun 16 10:55:07 UTC 2020
Hi,
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.
This is consistent with what I observe in MPFR with
#define MPFR_NEED_LONGLONG_H
#include "mpfr-test.h"
#include "invsqrt_limb.h"
int main (void)
{
mp_limb_t h, l;
sub_ddmmss (h, l, 0x4000000000000000, 0, 0x4000000000000000, 0);
printf ("%lx\n", h);
return 0;
}
which outputs ffffffffffffffff instead of 0.
--
Vincent Lefèvre <vincent at vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
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