Re: [boost] Boost Trac, random, No-Maintainer?

Subject: Re: [boost] Boost Trac, random, No-Maintainer?
From: Peter Dimov (lists_at_[hidden])
Date: 2017-10-08 15:47:51

> James E. King, III wrote:
>
> > For folks who like (potential) compiler optimization bugs:
> > https://svn.boost.org/trac10/ticket/13247
>
> Looking at the source of independent_bits, this jumps out at me:
>
>     S = (S << w0) + (u & y0_mask);
>
> and later
>
>     S = (S << (w0 + 1)) + (u & y1_mask);
>
> Shifts with a value more than the number of bits are undefined, and 
> debugging confirms that w0 is 32 in the failing tests.

Possible fix:

C:\Projects\boost-git\boost\libs\random>git diff
diff --git a/include/boost/random/independent_bits.hpp 
b/include/boost/random/independent_bits.hpp
index dec63b3..835ec32 100644
--- a/include/boost/random/independent_bits.hpp
+++ b/include/boost/random/independent_bits.hpp
@@ -159,7 +159,14 @@ public:
         BOOST_ASSERT(n0*w0 + (n - n0)*(w0 + 1) == w);

         result_type S = 0;
-        for(std::size_t k = 0; k < n0; ++k) {
+        if(0 < n0) {
+            base_unsigned u;
+            do {
+                u = detail::subtract<base_result_type>()(_base(), 
(_base.min)());
+            } while(u > base_unsigned(y0 - 1));
+            S = (u & y0_mask);
+        }
+        for(std::size_t k = 1; k < n0; ++k) {
             base_unsigned u;
             do {
                 u = detail::subtract<base_result_type>()(_base(), 
(_base.min)()
);

I also see (unrelated) variant=release errors on msvc-12.0/msvc-14.0, but 
haven't investigated them.

...failed testing.capture-output 
..\..\bin.v2\libs\random\test\test_knuth_b.test\msvc-12.0\release\threadapi-win32\threading-multi\test_knuth_b.run...
...failed testing.capture-output 
..\..\bin.v2\libs\random\test\test_ecuyer1988.test\msvc-12.0\release\threadapi-win32\threading-multi\test_ecuyer1988.run...
...failed testing.capture-output 
..\..\bin.v2\libs\random\test\test_independent_bits31.test\msvc-12.0\release\threadapi-win32\threading-multi\test_independent_bits31.run...
...failed testing.capture-output 
..\..\bin.v2\libs\random\test\test_ecuyer1988.test\msvc-14.0\release\threadapi-win32\threading-multi\test_ecuyer1988.run...
...failed testing.capture-output 
..\..\bin.v2\libs\random\test\test_knuth_b.test\msvc-14.0\release\threadapi-win32\threading-multi\test_knuth_b.run...
...failed testing.capture-output 
..\..\bin.v2\libs\random\test\test_independent_bits31.test\msvc-14.0\release\threadapi-win32\threading-multi\test_independent_bits31.run...