Reimplemented segmented_is_sorted_until to simplify and speed it up. The new implementation does an early check and there is no longer need to pass a boolean validity flag for the previous iterator.

Author: igaztanaga

include/boost/container/experimental/segmented_is_sorted_until.hpp

@@ -30,68 +30,42 @@ namespace detail_algo {
 
 // Recursive dispatch for `segmented_is_sorted_until`.
 //
-// Cross-segment state is carried by reference:
+// Cross-segment state is carried by value and returned in segduo<Iter, DeepIt>:
 //   - `prev`      : iterator (of the deepest non-segmented local type) pointing
-//                   at the most recently examined element.  Valid only when
-//                   `has_prev` is true.
-//   - `has_prev`  : becomes true the first time any element is seen.
+//                   at the most recently examined element.
 //
-// The segmented overload is the workhorse: it decomposes a range into
-// first / middle / last sub-ranges and dispatches each one back through
-// `sorted_until_rec`, which recurses if the local iterator is itself
-// segmented, until the non-segmented base case is reached.  `prev` and
-// `has_prev` thread through every level of recursion unchanged, which is how
-// the "last element of segment K" is compared against "first element of
-// segment K+1" even when K and K+1 live at different levels of the nesting.
+// All sorted_until_rec overloads return segduo<Iter, DeepIt> where .first is
+// the result iterator and .second is the updated prev value.  Passing prev by
+// value instead of by reference avoids pointer aliasing and lets the compiler
+// keep prev in a register throughout the inner loop.
 
 // (1) Non-segmented base case.
 //     At this level FwdIt == DeepIt (the deepest type computed from the top).
 template <class FwdIt, class Sent, class Comp, class DeepIt, class Cat>
-FwdIt sorted_until_rec
+segduo<FwdIt, DeepIt> sorted_until_rec
    (FwdIt first, Sent last, Comp comp,
-    DeepIt &prev_out, bool &has_prev,
-    const non_segmented_iterator_tag &, Cat)
+    DeepIt prev, const non_segmented_iterator_tag &, Cat)
 {
-   DeepIt prev = prev_out;
-   if (!has_prev) {
-      if (first == last)
-         return first;
-      prev = first;
-      has_prev = true;
-      ++first;
-   }
    for (; first != last; ++first) {
       if (comp(*first, *prev))
-         goto final_update;
-      prev = first;
+         break;
+      prev = detail_algo::deepest_local_iterator<FwdIt>::get(first);
    }
-   final_update:
-   prev_out = prev;
-   return first;
+   return segduo<FwdIt, DeepIt>(first, prev);
 }
 
 #if defined(BOOST_CONTAINER_SEGMENTED_LOOP_UNROLLING)
 
 // (1') Random-access unrolled variant of the non-segmented base case.
 template <class RAIter, class Comp, class DeepIt>
-RAIter sorted_until_rec
+segduo<RAIter, DeepIt> sorted_until_rec
    (RAIter first, RAIter last, Comp comp,
-    DeepIt &prev_out, bool &has_prev,
+    RAIter prev,
     const non_segmented_iterator_tag &, const std::random_access_iterator_tag &)
 {
    typedef typename iterator_traits<RAIter>::difference_type difference_type;
 
    difference_type n = last - first;
-   if (n == difference_type(0))
-      return last;
-
-   DeepIt prev = prev_out;
-   if (!has_prev) {
-      prev = first;
-      has_prev = true;
-      ++first;
-      --n;
-   }
 
    while (n >= difference_type(4)) {
       if (comp(*first, *prev))
@@ -130,43 +104,33 @@ RAIter sorted_until_rec
    }
 
    final_result:
-   prev_out = prev;
-   return first;
+   return segduo<RAIter, DeepIt>(first, prev);
 }
 
 #endif   //BOOST_CONTAINER_SEGMENTED_LOOP_UNROLLING
 
 // (2) Segmented iterator with a heterogeneous sentinel as end.
-//     Cannot locate `last`'s segment, so falls back to a simple linear loop
-//     using a local iterator as previous.  The `DeepIt &` / `has_prev &` are
-//     not used here because this overload is only ever invoked at the very
-//     top level (no cross-level state to propagate).
-template <class SegIter, class Sent, class Comp, class DeepIt, class Cat>
-typename algo_enable_if_c<is_sentinel<Sent, SegIter>::value, SegIter>::type
+//     Cannot locate `last`'s segment, so falls back to a simple linear loop.
+template <class SegIter, class Sent, class Comp, class DeepIt, class Tag, class Cat>
+BOOST_CONTAINER_FORCEINLINE
+typename algo_enable_if_c< is_sentinel<Sent, SegIter>::value
+                         , segduo<SegIter, DeepIt> >::type
 sorted_until_rec
    (SegIter first, Sent last, Comp comp,
-    DeepIt &, bool &,
-    const segmented_iterator_tag &, Cat)
+    DeepIt prev, const Tag &, const Cat &cat)
 {
-   if (first != last) {
-      SegIter prev_local = first;
-      for (; ++first != last; ) {
-         if (comp(*first, *prev_local))
-            return first;
-         prev_local = first;
-      }
-   }
-   return first;
+   return sorted_until_rec
+      (first, last, comp, prev, non_segmented_iterator_tag(), cat);
 }
 
 // (3) Segmented iterator with a matching segmented iterator as end.
 //     Recursive: decomposes the range and dispatches each sub-range back to
 //     `sorted_until_rec`, so recursive segmentation is exploited at every
-//     level.  `prev` / `has_prev` are threaded unchanged.
+//     level.  `prev` is threaded by value through the segduo return type.
 template <class SegIter, class Comp, class DeepIt, class Cat>
-SegIter sorted_until_rec
+segduo<SegIter, DeepIt> sorted_until_rec
    (SegIter first, SegIter last, Comp comp,
-    DeepIt &prev, bool &has_prev,
+    DeepIt prev,
     const segmented_iterator_tag &, Cat)
 {
    typedef segmented_iterator_traits<SegIter>                             traits;
@@ -175,44 +139,35 @@ SegIter sorted_until_rec
    typedef typename segmented_iterator_traits<local_iterator>::is_segmented_iterator
                                                                           is_local_seg_t;
    typedef typename iterator_traits<local_iterator>::iterator_category    local_cat_t;
+   typedef segduo<local_iterator, DeepIt>                                 local_result_t;
 
    segment_iterator       sfirst = traits::segment(first);
    const segment_iterator slast  = traits::segment(last);
 
    if (sfirst == slast) {
       const local_iterator ll = traits::local(last);
-      const local_iterator lr = (sorted_until_rec)
-         (traits::local(first), ll, comp, prev, has_prev, is_local_seg_t(), local_cat_t());
-      if (lr != ll)
-         return traits::compose(sfirst, lr);
+      const local_result_t r = (sorted_until_rec)
+         (traits::local(first), ll, comp, prev, is_local_seg_t(), local_cat_t());
+      return segduo<SegIter, DeepIt>(traits::compose(sfirst, r.first), r.second);
    }
    else {
-      // First segment: from local(first) to end(sfirst)
-      {
-         const local_iterator le = traits::end(sfirst);
-         const local_iterator lr = (sorted_until_rec)
-            (traits::local(first), le, comp, prev, has_prev, is_local_seg_t(), local_cat_t());
-         if (lr != le)
-            return traits::compose(sfirst, lr);
-      }
-      // Middle segments
+      local_iterator le = traits::end(sfirst);
+      local_result_t r = (sorted_until_rec)
+         (traits::local(first), le, comp, prev, is_local_seg_t(), local_cat_t());
+      if (r.first != le)
+         return segduo<SegIter, DeepIt>(traits::compose(sfirst, r.first), r.second);
+
       for (++sfirst; sfirst != slast; ++sfirst) {
-         const local_iterator le = traits::end(sfirst);
-         const local_iterator lr = (sorted_until_rec)
-            (traits::begin(sfirst), le, comp, prev, has_prev, is_local_seg_t(), local_cat_t());
-         if (lr != le)
-            return traits::compose(sfirst, lr);
-      }
-      // Last segment: from begin(slast) to local(last)
-      {
-         const local_iterator ll = traits::local(last);
-         const local_iterator lr = (sorted_until_rec)
-            (traits::begin(slast), ll, comp, prev, has_prev, is_local_seg_t(), local_cat_t());
-         if (lr != ll)
-            return traits::compose(sfirst, lr);
+         le = traits::end(sfirst);
+         r = (sorted_until_rec)
+            (traits::begin(sfirst), le, comp, r.second, is_local_seg_t(), local_cat_t());
+         if (r.first != le)
+            return segduo<SegIter, DeepIt>(traits::compose(sfirst, r.first), r.second);
       }
+      r = (sorted_until_rec)
+         (traits::begin(slast), traits::local(last), comp, r.second, is_local_seg_t(), local_cat_t());
+      return segduo<SegIter, DeepIt>(traits::compose(sfirst, r.first), r.second);
    }
-   return last;
 }
 
 } // namespace detail_algo
@@ -224,14 +179,18 @@ template <class FwdIt, class Sent, class Comp>
 BOOST_CONTAINER_FORCEINLINE
 FwdIt segmented_is_sorted_until(FwdIt first, Sent last, Comp comp)
 {
-   typedef segmented_iterator_traits<FwdIt>                          traits;
-   typedef typename detail_algo::deepest_local_iterator<FwdIt>::type deep_it;
-   deep_it prev = deep_it();
-   bool    has_prev = false;
+   if (first == last)
+      return first;
+
+   typedef segmented_iterator_traits<FwdIt>           traits;
+   typedef detail_algo::deepest_local_iterator<FwdIt> deep_it_helper;
+   typedef typename deep_it_helper::type              deep_it;
+
+   deep_it prev = deep_it_helper::get(first);
    return detail_algo::sorted_until_rec
-      (first, last, comp, prev, has_prev,
-       typename traits::is_segmented_iterator(),
-       typename iterator_traits<FwdIt>::iterator_category());
+      (++first, last, comp, prev,
+         typename traits::is_segmented_iterator(),
+         typename iterator_traits<FwdIt>::iterator_category()).first;
 }
 
 //! Returns an iterator to the first element that is less than its predecessor
@@ -249,4 +208,4 @@ FwdIt segmented_is_sorted_until(FwdIt first, Sent last)
 
 #include <boost/container/detail/config_end.hpp>
 
-#endif // BOOST_CONTAINER_EXPERIMENTAL_SEGMENTED_IS_SORTED_UNTIL_HPP
+#endif // BOOST_CONTAINER_EXPERIMENTAL_SEGMENTED_IS_SORTED_UNTIL_HPP