ibm-ldouble.c (__gcc_qdiv): Scale up arguments in case of small numerator and…

ibm-ldouble.c (__gcc_qdiv): Scale up arguments in case of small numerator and finite nonzero result.

libgcc:
	* config/rs6000/ibm-ldouble.c (__gcc_qdiv): Scale up arguments in
	case of small numerator and finite nonzero result.

gcc/testsuite:
	* gcc.target/powerpc/rs6000-ldouble-3.c: New test.

From-SVN: r206310

gcc/testsuite/ChangeLog

+2014-01-02  Joseph Myers  <joseph@codesourcery.com>
+
+	* gcc.target/powerpc/rs6000-ldouble-3.c: New test.
+
 2014-01-02  Marc Glisse  <marc.glisse@inria.fr>
 
 	PR c++/59641

gcc/testsuite/gcc.target/powerpc/rs6000-ldouble-3.c

+/* Test accuracy of long double division (glibc bug 15396).  */
+/* { dg-do run { target powerpc*-*-linux* powerpc*-*-darwin* powerpc*-*-aix* rs6000-*-* } } */
+/* { dg-options "-mlong-double-128" } */
+
+extern void exit (int);
+extern void abort (void);
+
+volatile long double a = 0x1p-1024L;
+volatile long double b = 0x3p-53L;
+volatile long double r;
+volatile long double expected = 0x1.55555555555555555555555555p-973L;
+
+int
+main (void)
+{
+  r = a / b;
+  /* Allow error up to 2ulp.  */
+  if (__builtin_fabsl (r - expected) > 0x1p-1073L)
+    abort ();
+  exit (0);
+}

libgcc/ChangeLog

+2014-01-02  Joseph Myers  <joseph@codesourcery.com>
+
+	* config/rs6000/ibm-ldouble.c (__gcc_qdiv): Scale up arguments in
+	case of small numerator and finite nonzero result.
+
 2014-01-02  Richard Sandiford  <rdsandiford@googlemail.com>
 
 	Update copyright years

libgcc/config/rs6000/ibm-ldouble.c

@@ -190,7 +190,16 @@ __gcc_qdiv (double a, double b, double c, double d)
       || nonfinite (t))
     return t;
 
-  /* Finite nonzero result requires corrections to the highest order term.  */
+  /* Finite nonzero result requires corrections to the highest order
+     term.  These corrections require the low part of c * t to be
+     exactly represented in double.  */
+  if (fabs (a) <= 0x1p-969)
+    {
+      a *= 0x1p106;
+      b *= 0x1p106;
+      c *= 0x1p106;
+      d *= 0x1p106;
+    }
 
   s = c * t;                    /* (s,sigma) = c*t exactly.  */
   w = -(-b + d * t);	/* Written to get fnmsub for speed, but not