Fix incorrect rounding of trunc, truncf, truncl in some cases. Add a new test.

diff --git a/ChangeLog b/ChangeLog
index e2ec18d..6278fa0 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
 2007-10-12  Bruno Haible  <bruno@clisp.org>
 
+       * lib/trunc.c (FUNC): Avoid rounding errors for values near a power
+       of 2.
+       * tests/test-truncf2.c: New file.
+       * modules/truncf-tests: (Files, Depends-on, Makefile.am): Add new test.
+
        * tests/test-truncf1.c: Renamed from tests/test-truncf.c.
        * modules/truncf-tests: Update.
 

diff --git a/lib/trunc.c b/lib/trunc.c
index 090cf09..e8d3d87 100644 (file)
--- a/lib/trunc.c
+++ b/lib/trunc.c
@@ -65,21 +65,29 @@ FUNC (DOUBLE x)
 
   if (z > L_(0.0))
     {
-      /* Round to the next integer (nearest or up or down, doesn't matter).  */
-      z += TWO_MANT_DIG;
-      z -= TWO_MANT_DIG;
-      /* Enforce rounding down.  */
-      if (z > y)
-       z -= L_(1.0);
+      /* Avoid rounding errors for values near 2^k, where k >= MANT_DIG-1.  */
+      if (z < TWO_MANT_DIG)
+       {
+         /* Round to the next integer (nearest or up or down, doesn't matter).  */
+         z += TWO_MANT_DIG;
+         z -= TWO_MANT_DIG;
+         /* Enforce rounding down.  */
+         if (z > y)
+           z -= L_(1.0);
+       }
     }
   else if (z < L_(0.0))
     {
-      /* Round to the next integer (nearest or up or down, doesn't matter).  */
-      z -= TWO_MANT_DIG;
-      z += TWO_MANT_DIG;
-      /* Enforce rounding up.  */
-      if (z < y)
-       z += L_(1.0);
+      /* Avoid rounding errors for values near -2^k, where k >= MANT_DIG-1.  */
+      if (z > - TWO_MANT_DIG)
+       {
+         /* Round to the next integer (nearest or up or down, doesn't matter).  */
+         z -= TWO_MANT_DIG;
+         z += TWO_MANT_DIG;
+         /* Enforce rounding up.  */
+         if (z < y)
+           z += L_(1.0);
+       }
     }
   return z;
 }

diff --git a/modules/truncf-tests b/modules/truncf-tests
index a7b4d53..cece64b 100644 (file)
--- a/modules/truncf-tests
+++ b/modules/truncf-tests
@@ -1,14 +1,20 @@
 Files:
 tests/test-truncf1.c
+tests/test-truncf2.c
 
 Depends-on:
+float
+isnanf-nolibm
+stdbool
+stdint
 
 configure.ac:
 
 Makefile.am:
-TESTS += test-truncf1
-check_PROGRAMS += test-truncf1
+TESTS += test-truncf1 test-truncf2
+check_PROGRAMS += test-truncf1 test-truncf2
 test_truncf1_LDADD = $(LDADD) @TRUNCF_LIBM@
+test_truncf2_LDADD = $(LDADD) @TRUNCF_LIBM@
 
 License:
 LGPL

diff --git a/tests/test-truncf2.c b/tests/test-truncf2.c
new file mode 100644 (file)
index 0000000..c511cb1
--- /dev/null
+++ b/tests/test-truncf2.c
@@ -0,0 +1,166 @@
+/* Test of rounding towards zero.
+   Copyright (C) 2007 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2007.  */
+
+#include <config.h>
+
+#include <math.h>
+
+#include <float.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "isnanf.h"
+
+#define ASSERT(expr) \
+  do                                                                        \
+    {                                                                       \
+      if (!(expr))                                                          \
+        {                                                                   \
+          fprintf (stderr, "%s:%d: assertion failed\n", __FILE__, __LINE__); \
+          abort ();                                                         \
+        }                                                                   \
+    }                                                                       \
+  while (0)
+
+
+/* The reference implementation, taken from lib/trunc.c.  */
+
+#define DOUBLE float
+#define MANT_DIG FLT_MANT_DIG
+#define L_(literal) literal##f
+
+/* 2^(MANT_DIG-1).  */
+static const DOUBLE TWO_MANT_DIG =
+  /* Assume MANT_DIG <= 5 * 31.
+     Use the identity
+       n = floor(n/5) + floor((n+1)/5) + ... + floor((n+4)/5).  */
+  (DOUBLE) (1U << ((MANT_DIG - 1) / 5))
+  * (DOUBLE) (1U << ((MANT_DIG - 1 + 1) / 5))
+  * (DOUBLE) (1U << ((MANT_DIG - 1 + 2) / 5))
+  * (DOUBLE) (1U << ((MANT_DIG - 1 + 3) / 5))
+  * (DOUBLE) (1U << ((MANT_DIG - 1 + 4) / 5));
+
+DOUBLE
+truncf_reference (DOUBLE x)
+{
+  /* The use of 'volatile' guarantees that excess precision bits are dropped
+     at each addition step and before the following comparison at the caller's
+     site.  It is necessary on x86 systems where double-floats are not IEEE
+     compliant by default, to avoid that the results become platform and compiler
+     option dependent.  'volatile' is a portable alternative to gcc's
+     -ffloat-store option.  */
+  volatile DOUBLE y = x;
+  volatile DOUBLE z = y;
+
+  if (z > L_(0.0))
+    {
+      /* Avoid rounding errors for values near 2^k, where k >= MANT_DIG-1.  */
+      if (z < TWO_MANT_DIG)
+       {
+         /* Round to the next integer (nearest or up or down, doesn't matter).  */
+         z += TWO_MANT_DIG;
+         z -= TWO_MANT_DIG;
+         /* Enforce rounding down.  */
+         if (z > y)
+           z -= L_(1.0);
+       }
+    }
+  else if (z < L_(0.0))
+    {
+      /* Avoid rounding errors for values near -2^k, where k >= MANT_DIG-1.  */
+      if (z > - TWO_MANT_DIG)
+       {
+         /* Round to the next integer (nearest or up or down, doesn't matter).  */
+         z -= TWO_MANT_DIG;
+         z += TWO_MANT_DIG;
+         /* Enforce rounding up.  */
+         if (z < y)
+           z += L_(1.0);
+       }
+    }
+  return z;
+}
+
+
+/* Test for equality.  */
+static int
+equal (DOUBLE x, DOUBLE y)
+{
+  return (isnanf (x) ? isnanf (y) : x == y);
+}
+
+/* Test whether the result for a given argument is correct.  */
+static bool
+correct_result_p (DOUBLE x, DOUBLE result)
+{
+  return
+    (x >= 0
+     ? (x < 1 ? result == L_(0.0) :
+       x - 1 < x ? result <= x && result > x - 1 :
+       equal (result, x))
+     : (x > -1 ? result == L_(0.0) :
+       x + 1 > x ? result >= x && result < x + 1 :
+       equal (result, x)));
+}
+
+/* Test the function for a given argument.  */
+static int
+check (float x)
+{
+  /* If the reference implementation is incorrect, bail out immediately.  */
+  float reference = truncf_reference (x);
+  ASSERT (correct_result_p (x, reference));
+  /* If the actual implementation is wrong, return an error code.  */
+  {
+    float result = truncf (x);
+    if (correct_result_p (x, result))
+      return 0;
+    else
+      {
+       fprintf (stderr, "truncf %g(%a) = %g(%a) or %g(%a)?\n",
+                x, x, reference, reference, result, result);
+       return 1;
+      }
+  }
+}
+
+#define NUM_HIGHBITS 12
+#define NUM_LOWBITS 4
+
+int
+main ()
+{
+  unsigned int highbits;
+  unsigned int lowbits;
+  int error = 0;
+  for (highbits = 0; highbits < (1 << NUM_HIGHBITS); highbits++)
+    for (lowbits = 0; lowbits < (1 << NUM_LOWBITS); lowbits++)
+      {
+       /* Combine highbits and lowbits into a floating-point number,
+          sign-extending the lowbits to 32-NUM_HIGHBITS bits.  */
+       union { float f; uint32_t i; } janus;
+       janus.i = ((uint32_t) highbits << (32 - NUM_HIGHBITS))
+                 | ((uint32_t) ((int32_t) ((uint32_t) lowbits << (32 - NUM_LOWBITS))
+                                >> (32 - NUM_LOWBITS - NUM_HIGHBITS))
+                    >> NUM_HIGHBITS);
+       error |= check (janus.f);
+      }
+  return (error ? 1 : 0);
+}