[WIP] perf(fill): make regular::index branch-free

include/boost/histogram/axis/regular.hpp

@@ -314,16 +314,18 @@ class regular : public iterator_mixin<regular<Value, Transform, MetaData, Option
         return static_cast<index_type>(z * size());
       }
     } else {
-      if (z < 1) {
-        if (z >= 0)
-          return static_cast<index_type>(z * size());
-        else
-          return -1;
-      }
+      // Branch-free so that random data does not mispredict. The clamped value
+      // keeps the conversion to int in range for any input. fmin(NaN, 1) is 1,
+      // so NaN maps to size(); the order of fmin and fmax matters.
+      const bool below = z < 0;
+      const auto zc = std::fmax(std::fmin(z, decltype(z){1}), decltype(z){0});
+      index_type i = static_cast<index_type>(zc * size());
+      i = below ? -1 : i;
       // upper edge of last bin is inclusive if overflow bin is not present
-      if (!options_type::test(option::overflow) && z == 1) return size() - 1;
+      if (!options_type::test(option::overflow)) i = z == 1 ? size() - 1 : i;
+      return i; // size() if x is NaN
     }
-    return size(); // also returned if x is NaN
+    return size();
   }
 
   /// Returns index and shift (if axis has grown) for the passed argument.

include/boost/histogram/detail/fill_n.hpp

@@ -219,7 +219,8 @@ void fill_n_storage(S& s, const Index idx, weight_type<T>&& w, Ts&&... ps) noexc
 template <class Index, class S, class A, class T, class... Ts>
 void fill_n_nd(const std::size_t offset, S& storage, A& axes, const std::size_t vsize,
                const T* values, Ts&&... ts) {
-  constexpr std::size_t buffer_size = 1ul << 14;
+  // 32 kB of indices; a larger buffer spills out of L1 and slows the passes below
+  constexpr std::size_t buffer_size = 1ul << 12;
   Index indices[buffer_size];
 
   /*