add comment about hint

Author: david-cortes-intel

cpp/daal/src/algorithms/dtrees/forest/regression/df_regression_train_dense_default_impl.i

@@ -219,6 +219,12 @@ void OrderedRespHelperBest<algorithmFPType, cpu>::calcImpurity(const IndexType *
             {
                 const size_t iStart = iMain * simdBatchSize;
 #if defined(__GNUC__) && !defined(__clang__)
+                // Note: GCC is unable to determine that the pointer offset plus the
+                // vectorized section will be within the limits of pointer ranges
+                // from the start of the array, and without this hint, ends up issuing
+                // a warning about undefined behavior when loops exceed 2^63. This
+                // attribute leads to a compilation error with LLVM-based compilers,
+                // so it's defined conditionally regardless of compiler support.
                 __attribute__((__assume__(iStart < std::numeric_limits<std::ptrdiff_t>::max())));
 #endif
                 const auto aIdxStart        = aIdx + iStart;