Merge pull request #1453 from tinko92/fix/index-content-fp-contract-off

Prevent FP contractions to FMA in index/.../content.hpp to avoid numerical instability in rtree

Author: tinko92

include/boost/geometry/index/detail/algorithms/content.hpp

@@ -16,6 +16,8 @@
 #ifndef BOOST_GEOMETRY_INDEX_DETAIL_ALGORITHMS_CONTENT_HPP
 #define BOOST_GEOMETRY_INDEX_DETAIL_ALGORITHMS_CONTENT_HPP
 
+#include <boost/config.hpp>
+
 #include <boost/geometry/core/access.hpp>
 #include <boost/geometry/core/coordinate_dimension.hpp>
 #include <boost/geometry/core/coordinate_type.hpp>
@@ -26,6 +28,18 @@
 
 namespace boost { namespace geometry { namespace index { namespace detail {
 
+#if defined(BOOST_GCC)
+// The following pragmas set fp-contract=off for GCC, to prevent surprising and
+// subtle numerical behaviour changes as a result of cross-statement/function
+// FMA contractions which can result in e.g. content(b1) - content(b2) != 0 
+// (or < 0) for content(b1) == content(b2) (or >=), due to the fp-contract=fast
+// default from at least GCC 4.4 to present GCC 15.2. This avoids Github issue
+// #1452. Safe to remove after numerically robust handling of degenerate cases
+// is ensured at all index::detail::content(box) call sites.
+#pragma GCC push_options
+#pragma GCC optimize ("fp-contract=off")
+#endif
+
 template <typename Indexable>
 struct default_content_result
 {
@@ -98,6 +112,10 @@ typename default_content_result<Indexable>::type content(Indexable const& b)
             >::apply(b);
 }
 
+#if defined(BOOST_GCC)
+#pragma GCC pop_options
+#endif
+
 }}}} // namespace boost::geometry::index::detail
 
 #endif // BOOST_GEOMETRY_INDEX_DETAIL_ALGORITHMS_CONTENT_HPP