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

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>

diff --git a/math/libm-test-remainder.inc b/math/libm-test-remainder.inc
index 0b846b2ddf9a2b7e875f36dc9eff60e432ae1f60..02e1e220dbdadc32bb4a331392455cee019b3691 100644 (file)
--- a/math/libm-test-remainder.inc
+++ b/math/libm-test-remainder.inc
@@ -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

diff --git a/sysdeps/ieee754/flt-32/e_remainderf.c b/sysdeps/ieee754/flt-32/e_remainderf.c
index 81781363af664af8ff2b85216298c88afd190305..bb066b7162fde0d5b87a6ad97e34ab6603530769 100644 (file)
--- a/sysdeps/ieee754/flt-32/e_remainderf.c
+++ b/sysdeps/ieee754/flt-32/e_remainderf.c
@@ -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;
 }

diff --git a/sysdeps/ieee754/ldbl-128/e_remainderl.c b/sysdeps/ieee754/ldbl-128/e_remainderl.c
index 07a15c24597d0d19210c0bbc615f7c13c33ed8b3..1e8605f258b26e0332342fb302657fe2db7d30f9 100644 (file)
--- a/sysdeps/ieee754/ldbl-128/e_remainderl.c
+++ b/sysdeps/ieee754/ldbl-128/e_remainderl.c
@@ -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;
 }