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[PATCH] BZ#887 logb[l](1) shows negative sign on zero inFE_DOWNWARD rounding mode


On POWER4 PowerPC32 for logb(1) and POWER[4|5|6] PowerPC32 for logbl(1)
in FE_DOWNWARD rounding mode the result is -0.0, which is contrary to
IEEE 754-2008 (section 5.3.3).

This is bugzilla bug #887:

http://sourceware.org/bugzilla/show_bug.cgi?id=887

This is due to the nature of the Power Architecture fsub instruction
under hardware rounding mode FE_DOWNWARD.

GCC recently introduced the ability to use the fcfid instruction in
32-bit mode on power4, which fixes the issue for logb(1) and expanded
the use of fcfid for power6 which corrected the issue for logbl(1).

r186387 | meissner | 2012-04-12 13:10:27 -0400 (Thu, 12 Apr 2012) | 16 lines
[gcc]
2012-04-11  Michael Meissner  <meissner@linux.vnet.ibm.com>
         PR target/52775
         * config/rs6000/rs6000.h (TARGET_FCFID): Add TARGET_PPC_GPOPT to
         the list of options to enable the FCFID instruction.
         (TARGET_EXTRA_BUILTINS): Adjust comment.

The included patch adds a test case to the libm test suite to detect
this issue if the compiler ever regresses.

I couldn't reproduce a similar nextafter() failure per the initial bug
report so I assume it's been fixed.

Ryan S. Arnold
IBM Linux Technology Center

2012-04-26  Ryan S. Arnold  <rsa@linux.vnet.ibm.com>

	[BZ #887]
	* math/libm-test.inc (logb_test_downward): New test to expose
	erroneous negative sign on -0.0 result of logb[l](1) in FE_DOWNWARD
	rounding mode.

diff --git a/math/libm-test.inc b/math/libm-test.inc
index e0ac613..81de85b 100644
--- a/math/libm-test.inc
+++ b/math/libm-test.inc
@@ -5101,6 +5101,40 @@ logb_test (void)
   END (logb);
 }
 
+static void
+logb_test_downward (void)
+{
+  int save_round_mode;
+  errno = 0;
+
+  FUNC(logb) (0);
+  if (errno == ENOSYS)
+    /* Function not implemented.  */
+    return;
+
+  START (logb_downward);
+
+  save_round_mode = fegetround ();
+
+  if (!fesetround (FE_DOWNWARD))
+    {
+
+      /* IEEE 754-2008 says (section 5.3.3) that "logB(1) is +0.".  Libm
+       * should not return -0 from logb in any rounding mode.  PowerPC32 has
+       * failed with this test for power4 logb (and logbl on all PowerPC
+       * platforms) in the past due to instruction selection.  GCC PR 52775
+       * provides the availability of the fcfid insn in 32-bit mode which
+       * eliminates the use of fsub in this instance and prevents the negative
+       * signed 0.0.  */
+
+      /* BZ #887 .*/
+      TEST_f_f (logb, 1.000e+0, plus_zero);
+    }
+
+  fesetround (save_round_mode);
+
+  END (logb_downward);
+}
 
 static void
 lround_test (void)
@@ -8210,6 +8244,7 @@ main (int argc, char **argv)
   log1p_test ();
   log2_test ();
   logb_test ();
+  logb_test_downward ();
   modf_test ();
   ilogb_test ();
   scalb_test ();



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