reduce register footprint

fix #2382

Rewrite UniformElements iterator to reduce the register footprint.

- avoid multiple return within a function
- reduce the iterator state size by one element

include/alpaka/exec/UniformElements.hpp

@@ -129,12 +129,11 @@ namespace alpaka
                 friend class UniformElementsAlong;
 
                 ALPAKA_FN_ACC inline const_iterator(Idx elements, Idx stride, Idx extent, Idx first)
-                    : elements_{elements}
-                    , stride_{stride}
-                    , extent_{extent}
-                    , first_{std::min(first, extent)}
-                    , index_{first_}
-                    , range_{std::min(first + elements, extent)}
+                    : extent_{extent}
+                    , elements_{elements} // we need to reduce the stride by on element range because index_ is later
+                                          // increased with each increment
+                    , stride_{stride - elements}
+                    , index_{std::min(first, extent)}
                 {
                 }
 
@@ -148,21 +147,16 @@ namespace alpaka
                 ALPAKA_FN_ACC inline const_iterator& operator++()
                 {
                     // increment the index along the elements processed by the current thread
+                    ++indexElem_;
                     ++index_;
-                    if(index_ < range_)
-                        return *this;
-
-                    // increment the thread index with the grid stride
-                    first_ += stride_;
-                    index_ = first_;
-                    range_ = std::min(first_ + elements_, extent_);
-                    if(index_ < extent_)
-                        return *this;
+                    if(indexElem_ >= elements_)
+                    {
+                        indexElem_ = Idx{0};
+                        index_ += stride_;
+                    }
+                    if(index_ >= extent_)
+                        index_ = extent_;
 
-                    // the iterator has reached or passed the end of the extent, clamp it to the extent
-                    first_ = extent_;
-                    index_ = extent_;
-                    range_ = extent_;
                     return *this;
                 }
 
@@ -176,7 +170,7 @@ namespace alpaka
 
                 ALPAKA_FN_ACC inline bool operator==(const_iterator const& other) const
                 {
-                    return (index_ == other.index_) and (first_ == other.first_);
+                    return (*(*this) == *other);
                 }
 
                 ALPAKA_FN_ACC inline bool operator!=(const_iterator const& other) const
@@ -186,13 +180,12 @@ namespace alpaka
 
             private:
                 // non-const to support iterator copy and assignment
+                Idx extent_;
                 Idx elements_;
                 Idx stride_;
-                Idx extent_;
                 // modified by the pre/post-increment operator
-                Idx first_;
                 Idx index_;
-                Idx range_;
+                Idx indexElem_ = 0;
             };
 
         private: