[libcxx] r177826 - Marshall Clow found some divide-by-zero warnings with UBSan in rand's binomial_distribution test. This eliminates the divide-by-zeros and describes in comments the numerical difficulties the test is having. Each of the problematic tests are exploring edge cases of the distribution.

[Howard Hinnant]

Author: hhinnant
Date: Sat Mar 23 14:29:45 2013
New Revision: 177826

URL: http://llvm.org/viewvc/llvm-project?rev=177826&view=rev
Log:
Marshall Clow found some divide-by-zero warnings with UBSan in rand's binomial_distribution test.  This eliminates the divide-by-zeros and describes in comments the numerical difficulties the test is having.  Each of the problematic tests are exploring edge cases of the distribution.

Modified:
    libcxx/trunk/test/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.bin/eval.pass.cpp

Modified: libcxx/trunk/test/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.bin/eval.pass.cpp
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/test/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.bin/eval.pass.cpp?rev=177826&r1=177825&r2=177826&view=diff
==============================================================================
--- libcxx/trunk/test/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.bin/eval.pass.cpp (original)
+++ libcxx/trunk/test/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.bin/eval.pass.cpp Sat Mar 23 14:29:45 2013
@@ -177,15 +177,23 @@ int main()
         }
         var /= u.size();
         double dev = std::sqrt(var);
-        skew /= u.size() * dev * var;
-        kurtosis /= u.size() * var * var;
-        kurtosis -= 3;
+        // In this case:
+        //   skew     computes to 0./0. == nan
+        //   kurtosis computes to 0./0. == nan
+        //   x_skew     == inf
+        //   x_kurtosis == inf
+        //   These tests are commented out because UBSan warns about division by 0
+//        skew /= u.size() * dev * var;
+//        kurtosis /= u.size() * var * var;
+//        kurtosis -= 3;
         double x_mean = d.t() * d.p();
         double x_var = x_mean*(1-d.p());
-        double x_skew = (1-2*d.p()) / std::sqrt(x_var);
-        double x_kurtosis = (1-6*d.p()*(1-d.p())) / x_var;
+//        double x_skew = (1-2*d.p()) / std::sqrt(x_var);
+//        double x_kurtosis = (1-6*d.p()*(1-d.p())) / x_var;
         assert(mean == x_mean);
         assert(var == x_var);
+//        assert(skew == x_skew);
+//        assert(kurtosis == x_kurtosis);
     }
     {
         typedef std::binomial_distribution<> D;
@@ -215,15 +223,23 @@ int main()
         }
         var /= u.size();
         double dev = std::sqrt(var);
-        skew /= u.size() * dev * var;
-        kurtosis /= u.size() * var * var;
-        kurtosis -= 3;
+        // In this case:
+        //   skew     computes to 0./0. == nan
+        //   kurtosis computes to 0./0. == nan
+        //   x_skew     == -inf
+        //   x_kurtosis == inf
+        //   These tests are commented out because UBSan warns about division by 0
+//        skew /= u.size() * dev * var;
+//        kurtosis /= u.size() * var * var;
+//        kurtosis -= 3;
         double x_mean = d.t() * d.p();
         double x_var = x_mean*(1-d.p());
-        double x_skew = (1-2*d.p()) / std::sqrt(x_var);
-        double x_kurtosis = (1-6*d.p()*(1-d.p())) / x_var;
+//        double x_skew = (1-2*d.p()) / std::sqrt(x_var);
+//        double x_kurtosis = (1-6*d.p()*(1-d.p())) / x_var;
         assert(mean == x_mean);
         assert(var == x_var);
+//        assert(skew == x_skew);
+//        assert(kurtosis == x_kurtosis);
     }
     {
         typedef std::binomial_distribution<> D;
@@ -333,15 +349,23 @@ int main()
         }
         var /= u.size();
         double dev = std::sqrt(var);
-        skew /= u.size() * dev * var;
-        kurtosis /= u.size() * var * var;
-        kurtosis -= 3;
+        // In this case:
+        //   skew     computes to 0./0. == nan
+        //   kurtosis computes to 0./0. == nan
+        //   x_skew     == inf
+        //   x_kurtosis == inf
+        //   These tests are commented out because UBSan warns about division by 0
+//        skew /= u.size() * dev * var;
+//        kurtosis /= u.size() * var * var;
+//        kurtosis -= 3;
         double x_mean = d.t() * d.p();
         double x_var = x_mean*(1-d.p());
-        double x_skew = (1-2*d.p()) / std::sqrt(x_var);
-        double x_kurtosis = (1-6*d.p()*(1-d.p())) / x_var;
+//        double x_skew = (1-2*d.p()) / std::sqrt(x_var);
+//        double x_kurtosis = (1-6*d.p()*(1-d.p())) / x_var;
         assert(mean == x_mean);
         assert(var == x_var);
+//        assert(skew == x_skew);
+//        assert(kurtosis == x_kurtosis);
     }
     {
         typedef std::binomial_distribution<> D;
@@ -371,15 +395,23 @@ int main()
         }
         var /= u.size();
         double dev = std::sqrt(var);
-        skew /= u.size() * dev * var;
-        kurtosis /= u.size() * var * var;
-        kurtosis -= 3;
+        // In this case:
+        //   skew     computes to 0./0. == nan
+        //   kurtosis computes to 0./0. == nan
+        //   x_skew     == inf
+        //   x_kurtosis == inf
+        //   These tests are commented out because UBSan warns about division by 0
+//        skew /= u.size() * dev * var;
+//        kurtosis /= u.size() * var * var;
+//        kurtosis -= 3;
         double x_mean = d.t() * d.p();
         double x_var = x_mean*(1-d.p());
-        double x_skew = (1-2*d.p()) / std::sqrt(x_var);
-        double x_kurtosis = (1-6*d.p()*(1-d.p())) / x_var;
+//        double x_skew = (1-2*d.p()) / std::sqrt(x_var);
+//        double x_kurtosis = (1-6*d.p()*(1-d.p())) / x_var;
         assert(mean == x_mean);
         assert(var == x_var);
+//        assert(skew == x_skew);
+//        assert(kurtosis == x_kurtosis);
     }
     {
         typedef std::binomial_distribution<> D;
@@ -409,14 +441,22 @@ int main()
         }
         var /= u.size();
         double dev = std::sqrt(var);
-        skew /= u.size() * dev * var;
-        kurtosis /= u.size() * var * var;
-        kurtosis -= 3;
+        // In this case:
+        //   skew     computes to 0./0. == nan
+        //   kurtosis computes to 0./0. == nan
+        //   x_skew     == -inf
+        //   x_kurtosis == inf
+        //   These tests are commented out because UBSan warns about division by 0
+//        skew /= u.size() * dev * var;
+//        kurtosis /= u.size() * var * var;
+//        kurtosis -= 3;
         double x_mean = d.t() * d.p();
         double x_var = x_mean*(1-d.p());
-        double x_skew = (1-2*d.p()) / std::sqrt(x_var);
-        double x_kurtosis = (1-6*d.p()*(1-d.p())) / x_var;
+//        double x_skew = (1-2*d.p()) / std::sqrt(x_var);
+//        double x_kurtosis = (1-6*d.p()*(1-d.p())) / x_var;
         assert(mean == x_mean);
         assert(var == x_var);
+//        assert(skew == x_skew);
+//        assert(kurtosis == x_kurtosis);
     }
 }