Fix x86/x86_64 nextafterl incrementing negative subnormals (bug 20205).

The x86 / x86_64 implementation of nextafterl (also used for
nexttowardl) produces incorrect results (NaNs) when negative
subnormals, the low 32 bits of whose mantissa are zero, are
incremented towards zero.  This patch fixes this by disabling the
logic to decrement the exponent in that case.

Tested for x86_64 and x86.

	[BZ #20205]
	* sysdeps/i386/fpu/s_nextafterl.c (__nextafterl): Do not adjust
	exponent when incrementing negative subnormal with low mantissa
	word zero.
	* math/libm-test.inc (nextafter_test_data) [TEST_COND_intel96]:
	Add another test.

diff --git a/ChangeLog b/ChangeLog
index 94a95f3c0020e9e3034c297fd8fcc4d3a9a599ab..3e16edf9e884b1264ee165cab853946dedacdefa 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2016-06-03  Joseph Myers  <joseph@codesourcery.com>
+
+       [BZ #20205]
+       * sysdeps/i386/fpu/s_nextafterl.c (__nextafterl): Do not adjust
+       exponent when incrementing negative subnormal with low mantissa
+       word zero.
+       * math/libm-test.inc (nextafter_test_data) [TEST_COND_intel96]:
+       Add another test.
+
 2016-06-03  Florian Weimer  <fweimer@redhat.com>
 
        * libio/wstrops.c (_IO_wstr_overflow, enlarge_userbuf): Use

diff --git a/math/libm-test.inc b/math/libm-test.inc
index 8c64c8e7581db5a36bede39f064cde69ce5a3727..aaa01488ec771ebc19536c3537eb12591b3e792c 100644 (file)
--- a/math/libm-test.inc
+++ b/math/libm-test.inc
@@ -9965,6 +9965,10 @@ static const struct test_ff_f_data nextafter_test_data[] =
     TEST_ff_f (nextafter, -min_subnorm_value, 0, minus_zero, INEXACT_EXCEPTION|UNDERFLOW_EXCEPTION|ERRNO_ERANGE),
     TEST_ff_f (nextafter, -min_subnorm_value, minus_zero, minus_zero, INEXACT_EXCEPTION|UNDERFLOW_EXCEPTION|ERRNO_ERANGE),
 
+#if TEST_COND_intel96
+    TEST_ff_f (nextafter, -0x0.fffffffep-16382L, 0.0L, -0x0.fffffffdfffffffep-16382L, INEXACT_EXCEPTION|UNDERFLOW_EXCEPTION|ERRNO_ERANGE),
+#endif
+
 #if MANT_DIG >= 64
     // XXX Enable once gcc is fixed.
     //TEST_ff_f (nextafter, 0x0.00000040000000000000p-16385L, -0.1L, 0x0.0000003ffffffff00000p-16385L),

diff --git a/sysdeps/i386/fpu/s_nextafterl.c b/sysdeps/i386/fpu/s_nextafterl.c
index 188dc2129a33a4f20040f470451964534211c070..600ad7a8d3ef2e4817d74990cbe2d4975fe5f9a1 100644 (file)
--- a/sysdeps/i386/fpu/s_nextafterl.c
+++ b/sysdeps/i386/fpu/s_nextafterl.c
@@ -86,7 +86,7 @@ long double __nextafterl(long double x, long double y)
            if(esy>=0||(esx>esy||((esx==esy)&&(hx>hy||((hx==hy)&&(lx>ly)))))){
              /* x < y, x -= ulp */
                if(lx==0) {
-                   if (hx <= 0x80000000) {
+                   if (hx <= 0x80000000 && esx != 0xffff8000) {
                        esx -= 1;
                        hx = hx - 1;
                        if ((esx&0x7fff) > 0)