[PATCH] sysdeps/ieee754/flt-32: Fix remainderf() sign of zero for FE_DOWNWARD

[Sergei Zimmerman]

Single-precision remainderf() 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.

Original fix by: Bruce Evans <bde@FreeBSD.org> in FreeBSD's
a2ddfa5ea726c56dbf825763ad371c261b89b7c7.
---
 math/libm-test-remainder.inc          | 4 ++++
 sysdeps/ieee754/flt-32/e_remainderf.c | 3 +++
 2 files changed, 7 insertions(+)

diff --git a/math/libm-test-remainder.inc b/math/libm-test-remainder.inc
index 1a1d3e9937..f2b0104087 100644
--- 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),
+    TEST_ff_f (remainder, -0x1.003ffep-126, -0x1.8p-148, minus_zero, NO_INEXACT_EXCEPTION|ERRNO_UNCHANGED),
+    TEST_ff_f (remainder, 0x1.2c3224p+17, 0x1p-5, plus_zero, NO_INEXACT_EXCEPTION|ERRNO_UNCHANGED),
   };
 
 static void
diff --git a/sysdeps/ieee754/flt-32/e_remainderf.c b/sysdeps/ieee754/flt-32/e_remainderf.c
index 81781363af..7d773ab36d 100644
--- 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&0x7fffffff)==0) hx = 0;
 	SET_FLOAT_WORD(x,hx^sx);
 	return x;
 }
-- 
2.47.2