diff mbox

Fix clog / clog10 sign of zero result in round-downward mode (bug 16789)

Message ID Pine.LNX.4.64.1404011543430.24927@digraph.polyomino.org.uk
State New
Headers show

Commit Message

Joseph Myers April 1, 2014, 3:46 p.m. UTC 
This patch fixes bug 16789, incorrect sign of (real part) zero result
from clog and clog10 in round-downward mode, arising from that real
part being computed as 0 - 0.  To ensure that an underflow exception
occurred, the code used an underflowing value (the next term in the
series for log1p) in arithmetic computing the real part of the result,
yielding the problematic 0 - 0 computation in some cases even when the
mathematical result would be small but positive.  The patch changes
this code to use the math_force_eval approach to ensuring that an
underflowing computation actually occurs.  Tests of clog and clog10
are enabled in all rounding modes.

Tested x86_64 and x86 and ulps updated accordingly.

2014-04-01  Joseph Myers  <joseph@codesourcery.com>

	[BZ #16789]
	* math/s_clog.c (__clog): Use math_force_eval to ensure underflow
	instead of using underflowing value in computing result.
	* math/s_clog10.c (__clog10): Likewise.
	* math/s_clog10f.c (__clog10f): Likewise.
	* math/s_clog10l.c (__clog10l): Likewise.
	* math/s_clogf.c (__clogf): Likewise.
	* math/s_clogl.c (__clogl): Likewise.
	* math/libm-test.inc (clog_test): Use ALL_RM_TEST.
	(clog10_test): Likewise.
	* sysdeps/i386/fpu/libm-test-ulps: Update.
	* sysdeps/x86_64/fpu/libm-test-ulps: Likewise.

Comments

Andreas Jaeger April 2, 2014, 12:36 p.m. UTC | #1 
On 04/01/2014 05:46 PM, Joseph S. Myers wrote:
> This patch fixes bug 16789, incorrect sign of (real part) zero result
> from clog and clog10 in round-downward mode, arising from that real
> part being computed as 0 - 0.  To ensure that an underflow exception
> occurred, the code used an underflowing value (the next term in the
> series for log1p) in arithmetic computing the real part of the result,
> yielding the problematic 0 - 0 computation in some cases even when the
> mathematical result would be small but positive.  The patch changes
> this code to use the math_force_eval approach to ensuring that an
> underflowing computation actually occurs.  Tests of clog and clog10
> are enabled in all rounding modes.

Thanks,
Andreas
diff mbox

Patch

diff --git a/math/libm-test.inc b/math/libm-test.inc
index 19194f6..fe43185 100644
--- a/math/libm-test.inc
+++ b/math/libm-test.inc
@@ -6122,9 +6122,7 @@  static const struct test_c_c_data clog_test_data[] =
 static void
 clog_test (void)
 {
-  START (clog, 0);
-  RUN_TEST_LOOP_c_c (clog, clog_test_data, );
-  END_COMPLEX;
+  ALL_RM_TEST (clog, 0, clog_test_data, RUN_TEST_LOOP_c_c, END_COMPLEX);
 }
 
 
@@ -6184,9 +6182,7 @@  static const struct test_c_c_data clog10_test_data[] =
 static void
 clog10_test (void)
 {
-  START (clog10, 0);
-  RUN_TEST_LOOP_c_c (clog10, clog10_test_data, );
-  END_COMPLEX;
+  ALL_RM_TEST (clog10, 0, clog10_test_data, RUN_TEST_LOOP_c_c, END_COMPLEX);
 }
 
 
diff --git a/math/s_clog.c b/math/s_clog.c
index 8639868..077f8f2 100644
--- a/math/s_clog.c
+++ b/math/s_clog.c
@@ -68,12 +68,9 @@  __clog (__complex__ double x)
 	  double absy2 = absy * absy;
 	  if (absy2 <= DBL_MIN * 2.0)
 	    {
-#if __FLT_EVAL_METHOD__ == 0
-	      __real__ result = absy2 / 2.0 - absy2 * absy2 / 4.0;
-#else
-	      volatile double force_underflow = absy2 * absy2 / 4.0;
-	      __real__ result = absy2 / 2.0 - force_underflow;
-#endif
+	      double force_underflow = absy2 * absy2;
+	      __real__ result = absy2 / 2.0;
+	      math_force_eval (force_underflow);
 	    }
 	  else
 	    __real__ result = __log1p (absy2) / 2.0;
diff --git a/math/s_clog10.c b/math/s_clog10.c
index e757879..aa0537c 100644
--- a/math/s_clog10.c
+++ b/math/s_clog10.c
@@ -74,12 +74,9 @@  __clog10 (__complex__ double x)
 	  double absy2 = absy * absy;
 	  if (absy2 <= DBL_MIN * 2.0 * M_LN10)
 	    {
-#if __FLT_EVAL_METHOD__ == 0
-	      __real__ result = (absy2 / 2.0 - absy2 * absy2 / 4.0) * M_LOG10E;
-#else
-	      volatile double force_underflow = absy2 * absy2 / 4.0;
-	      __real__ result = (absy2 / 2.0 - force_underflow) * M_LOG10E;
-#endif
+	      double force_underflow = absy2 * absy2;
+	      __real__ result = absy2 * (M_LOG10E / 2.0);
+	      math_force_eval (force_underflow);
 	    }
 	  else
 	    __real__ result = __log1p (absy2) * (M_LOG10E / 2.0);
diff --git a/math/s_clog10f.c b/math/s_clog10f.c
index ca2cdf4..3403b6c 100644
--- a/math/s_clog10f.c
+++ b/math/s_clog10f.c
@@ -74,14 +74,9 @@  __clog10f (__complex__ float x)
 	  float absy2 = absy * absy;
 	  if (absy2 <= FLT_MIN * 2.0f * (float) M_LN10)
 	    {
-#if __FLT_EVAL_METHOD__ == 0
-	      __real__ result
-		= (absy2 / 2.0f - absy2 * absy2 / 4.0f) * (float) M_LOG10E;
-#else
-	      volatile float force_underflow = absy2 * absy2 / 4.0f;
-	      __real__ result
-		= (absy2 / 2.0f - force_underflow) * (float) M_LOG10E;
-#endif
+	      float force_underflow = absy2 * absy2;
+	      __real__ result = absy2 * ((float) M_LOG10E / 2.0f);
+	      math_force_eval (force_underflow);
 	    }
 	  else
 	    __real__ result = __log1pf (absy2) * ((float) M_LOG10E / 2.0f);
diff --git a/math/s_clog10l.c b/math/s_clog10l.c
index cdb5d61..fd86ecb 100644
--- a/math/s_clog10l.c
+++ b/math/s_clog10l.c
@@ -80,8 +80,11 @@  __clog10l (__complex__ long double x)
 	{
 	  long double absy2 = absy * absy;
 	  if (absy2 <= LDBL_MIN * 2.0L * M_LN10l)
-	    __real__ result
-	      = (absy2 / 2.0L - absy2 * absy2 / 4.0L) * M_LOG10El;
+	    {
+	      long double force_underflow = absy2 * absy2;
+	      __real__ result = absy2 * (M_LOG10El / 2.0);
+	      math_force_eval (force_underflow);
+	    }
 	  else
 	    __real__ result = __log1pl (absy2) * (M_LOG10El / 2.0L);
 	}
diff --git a/math/s_clogf.c b/math/s_clogf.c
index 79117df..b487334 100644
--- a/math/s_clogf.c
+++ b/math/s_clogf.c
@@ -68,12 +68,9 @@  __clogf (__complex__ float x)
 	  float absy2 = absy * absy;
 	  if (absy2 <= FLT_MIN * 2.0f)
 	    {
-#if __FLT_EVAL_METHOD__ == 0
-	      __real__ result = absy2 / 2.0f - absy2 * absy2 / 4.0f;
-#else
-	      volatile float force_underflow = absy2 * absy2 / 4.0f;
-	      __real__ result = absy2 / 2.0f - force_underflow;
-#endif
+	      float force_underflow = absy2 * absy2;
+	      __real__ result = absy2 / 2.0f;
+	      math_force_eval (force_underflow);
 	    }
 	  else
 	    __real__ result = __log1pf (absy2) / 2.0f;
diff --git a/math/s_clogl.c b/math/s_clogl.c
index bdf82c1..1b4a304 100644
--- a/math/s_clogl.c
+++ b/math/s_clogl.c
@@ -74,7 +74,11 @@  __clogl (__complex__ long double x)
 	{
 	  long double absy2 = absy * absy;
 	  if (absy2 <= LDBL_MIN * 2.0L)
-	    __real__ result = absy2 / 2.0L - absy2 * absy2 / 4.0L;
+	    {
+	      long double force_underflow = absy2 * absy2;
+	      __real__ result = absy2 / 2.0L;
+	      math_force_eval (force_underflow);
+	    }
 	  else
 	    __real__ result = __log1pl (absy2) / 2.0L;
 	}
diff --git a/sysdeps/i386/fpu/libm-test-ulps b/sysdeps/i386/fpu/libm-test-ulps
index a82de19..0252d5b 100644
--- a/sysdeps/i386/fpu/libm-test-ulps
+++ b/sysdeps/i386/fpu/libm-test-ulps
@@ -593,6 +593,102 @@  ifloat: 1
 ildouble: 2
 ldouble: 2
 
+Function: Real part of "clog10_downward":
+double: 2
+float: 2
+idouble: 2
+ifloat: 2
+ildouble: 4
+ldouble: 4
+
+Function: Imaginary part of "clog10_downward":
+double: 2
+float: 2
+idouble: 2
+ifloat: 2
+ildouble: 2
+ldouble: 2
+
+Function: Real part of "clog10_towardzero":
+double: 2
+float: 1
+idouble: 2
+ifloat: 1
+ildouble: 4
+ldouble: 4
+
+Function: Imaginary part of "clog10_towardzero":
+double: 2
+float: 2
+idouble: 2
+ifloat: 2
+ildouble: 2
+ldouble: 2
+
+Function: Real part of "clog10_upward":
+double: 2
+float: 2
+idouble: 2
+ifloat: 2
+ildouble: 4
+ldouble: 4
+
+Function: Imaginary part of "clog10_upward":
+double: 2
+float: 2
+idouble: 2
+ifloat: 2
+ildouble: 2
+ldouble: 2
+
+Function: Real part of "clog_downward":
+double: 2
+float: 1
+idouble: 2
+ifloat: 1
+ildouble: 2
+ldouble: 2
+
+Function: Imaginary part of "clog_downward":
+double: 1
+float: 1
+idouble: 1
+ifloat: 1
+ildouble: 1
+ldouble: 1
+
+Function: Real part of "clog_towardzero":
+double: 2
+float: 1
+idouble: 2
+ifloat: 1
+ildouble: 2
+ldouble: 2
+
+Function: Imaginary part of "clog_towardzero":
+double: 1
+float: 1
+idouble: 1
+ifloat: 1
+ildouble: 1
+ldouble: 1
+
+Function: Real part of "clog_upward":
+double: 1
+float: 1
+idouble: 1
+ifloat: 1
+ildouble: 2
+ldouble: 2
+
+Function: Imaginary part of "clog_upward":
+double: 1
+float: 1
+idouble: 1
+ifloat: 1
+ildouble: 1
+ldouble: 1
+
 Function: "cos":
 ildouble: 1
 ldouble: 1
diff --git a/sysdeps/x86_64/fpu/libm-test-ulps b/sysdeps/x86_64/fpu/libm-test-ulps
index e2fa9b2..1b6ebe2 100644
--- a/sysdeps/x86_64/fpu/libm-test-ulps
+++ b/sysdeps/x86_64/fpu/libm-test-ulps
@@ -624,6 +624,102 @@  ifloat: 1
 ildouble: 2
 ldouble: 2
 
+Function: Real part of "clog10_downward":
+double: 3
+float: 3
+idouble: 3
+ifloat: 3
+ildouble: 4
+ldouble: 4
+
+Function: Imaginary part of "clog10_downward":
+double: 3
+float: 2
+idouble: 3
+ifloat: 2
+ildouble: 2
+ldouble: 2
+
+Function: Real part of "clog10_towardzero":
+double: 3
+float: 2
+idouble: 3
+ifloat: 2
+ildouble: 4
+ldouble: 4
+
+Function: Imaginary part of "clog10_towardzero":
+double: 3
+float: 2
+idouble: 3
+ifloat: 2
+ildouble: 2
+ldouble: 2
+
+Function: Real part of "clog10_upward":
+double: 4
+float: 3
+idouble: 4
+ifloat: 3
+ildouble: 4
+ldouble: 4
+
+Function: Imaginary part of "clog10_upward":
+double: 2
+float: 2
+idouble: 2
+ifloat: 2
+ildouble: 2
+ldouble: 2
+
+Function: Real part of "clog_downward":
+double: 2
+float: 2
+idouble: 2
+ifloat: 2
+ildouble: 2
+ldouble: 2
+
+Function: Imaginary part of "clog_downward":
+double: 1
+float: 2
+idouble: 1
+ifloat: 2
+ildouble: 1
+ldouble: 1
+
+Function: Real part of "clog_towardzero":
+double: 2
+float: 2
+idouble: 2
+ifloat: 2
+ildouble: 2
+ldouble: 2
+
+Function: Imaginary part of "clog_towardzero":
+double: 1
+float: 2
+idouble: 1
+ifloat: 2
+ildouble: 1
+ldouble: 1
+
+Function: Real part of "clog_upward":
+double: 2
+float: 1
+idouble: 2
+ifloat: 1
+ildouble: 2
+ldouble: 2
+
+Function: Imaginary part of "clog_upward":
+double: 2
+float: 2
+idouble: 2
+ifloat: 2
+ildouble: 1
+ldouble: 1
+
 Function: "cos":
 ildouble: 1
 ldouble: 1