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sysdeps/ieee754: Fix remainder sign of zero for FE_DOWNWARD (BZ #32711)

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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>
This commit is contained in:
Sergei Zimmerman 2025-02-25 18:05:40 +00:00 • committed by Adhemerval Zanella
parent 2fe5e2af09
commit 9e51ae3cd0
3 changed files with 11 additions and 1 deletions
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4
math/libm-test-remainder.inc
View file

@ -173,6 +173,10 @@ static const struct test_ff_f_data remainder_test_data[] =
TEST_ff_f (remainder, 2, -1, plus_zero, NO_INEXACT_EXCEPTION|ERRNO_UNCHANGED),
TEST_ff_f (remainder, -2, 1, minus_zero, NO_INEXACT_EXCEPTION|ERRNO_UNCHANGED),
TEST_ff_f (remainder, -2, -1, minus_zero, NO_INEXACT_EXCEPTION|ERRNO_UNCHANGED),
TEST_ff_f (remainder, 0x1.08001cp-2, 0x1p-24, plus_zero, NO_INEXACT_EXCEPTION|ERRNO_UNCHANGED|XFAIL_ROUNDING_IBM128_LIBGCC),
TEST_ff_f (remainder, -0x1.003ffep-126, -0x1.8p-148, minus_zero, NO_INEXACT_EXCEPTION|ERRNO_UNCHANGED|XFAIL_ROUNDING_IBM128_LIBGCC),
TEST_ff_f (remainder, 0x1.2c3224p+17, 0x1p-5, plus_zero, NO_INEXACT_EXCEPTION|ERRNO_UNCHANGED|XFAIL_ROUNDING_IBM128_LIBGCC),
};
static void

3
sysdeps/ieee754/flt-32/e_remainderf.c
View file

@ -56,6 +56,9 @@ __ieee754_remainderf(float x, float p)
}
}
GET_FLOAT_WORD(hx,x);
/* Make sure x is not -0. This can occur only when x = p
and rounding direction is towards negative infinity. */
if (hx==0x80000000) hx = 0;
SET_FLOAT_WORD(x,hx^sx);
return x;
}

5
sysdeps/ieee754/ldbl-128/e_remainderl.c
View file

@ -64,7 +64,10 @@ __ieee754_remainderl(_Float128 x, _Float128 p)
if(x>=p_half) x -= p;
}
}
GET_LDOUBLE_MSW64(hx,x);
GET_LDOUBLE_WORDS64(hx,lx,x);
/* Make sure x is not -0. This can occur only when x = p
and rounding direction is towards negative infinity. */
if ((hx==0x8000000000000000ULL)&&(lx==0)) hx = 0;
SET_LDOUBLE_MSW64(x,hx^sx);
return x;
}