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Single-precision remainderf() and quad-precision remainderl() implementation derived from Sun is affected by an issue when the result is +-0. IEEE754 requires that if remainder(x, y) = 0, its sign shall be that of x regardless of the rounding direction. The implementation seems to have assumed that x - x = +0 in all rounding modes, which is not the case. When rounding direction is roundTowardNegative the sign of an exact zero sum (or difference) is −0. Regression tests that triggered this erroneous behavior are added to math/libm-test-remainder.inc. Tested for cross riscv64 and powerpc. Original fix by: Bruce Evans <bde@FreeBSD.org> in FreeBSD's a2ddfa5ea726c56dbf825763ad371c261b89b7c7. Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
74 lines
1.9 KiB
C
74 lines
1.9 KiB
C
/* e_fmodl.c -- long double version of e_fmod.c.
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*/
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/*
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* ====================================================
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* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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*
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* Developed at SunPro, a Sun Microsystems, Inc. business.
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* Permission to use, copy, modify, and distribute this
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* software is freely granted, provided that this notice
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* is preserved.
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* ====================================================
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*/
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/* __ieee754_remainderl(x,p)
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* Return :
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* returns x REM p = x - [x/p]*p as if in infinite
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* precise arithmetic, where [x/p] is the (infinite bit)
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* integer nearest x/p (in half way case choose the even one).
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* Method :
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* Based on fmodl() return x-[x/p]chopped*p exactlp.
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*/
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#include <math.h>
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#include <math_private.h>
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#include <libm-alias-finite.h>
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static const _Float128 zero = 0;
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_Float128
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__ieee754_remainderl(_Float128 x, _Float128 p)
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{
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int64_t hx,hp;
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uint64_t sx,lx,lp;
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_Float128 p_half;
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GET_LDOUBLE_WORDS64(hx,lx,x);
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GET_LDOUBLE_WORDS64(hp,lp,p);
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sx = hx&0x8000000000000000ULL;
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hp &= 0x7fffffffffffffffLL;
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hx &= 0x7fffffffffffffffLL;
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/* purge off exception values */
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if((hp|lp)==0) return (x*p)/(x*p); /* p = 0 */
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if((hx>=0x7fff000000000000LL)|| /* x not finite */
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((hp>=0x7fff000000000000LL)&& /* p is NaN */
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(((hp-0x7fff000000000000LL)|lp)!=0)))
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return (x*p)/(x*p);
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if (hp<=0x7ffdffffffffffffLL) x = __ieee754_fmodl(x,p+p); /* now x < 2p */
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if (((hx-hp)|(lx-lp))==0) return zero*x;
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x = fabsl(x);
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p = fabsl(p);
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if (hp<0x0002000000000000LL) {
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if(x+x>p) {
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x-=p;
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if(x+x>=p) x -= p;
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}
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} else {
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p_half = L(0.5)*p;
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if(x>p_half) {
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x-=p;
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if(x>=p_half) x -= p;
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}
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}
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GET_LDOUBLE_WORDS64(hx,lx,x);
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/* Make sure x is not -0. This can occur only when x = p
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and rounding direction is towards negative infinity. */
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if ((hx==0x8000000000000000ULL)&&(lx==0)) hx = 0;
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SET_LDOUBLE_MSW64(x,hx^sx);
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return x;
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}
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libm_alias_finite (__ieee754_remainderl, __remainderl)
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