Improve performance of slice_strings for long strings

cpp/benchmarks/string/slice.cpp

@@ -43,7 +43,7 @@ static void BM_slice(benchmark::State& state, slice_type rt)
     cudf::type_id::STRING, distribution_id::NORMAL, 0, max_str_length);
   auto const column = create_random_column(cudf::type_id::STRING, row_count{n_rows}, profile);
   cudf::strings_column_view input(column->view());
-  auto starts_itr = thrust::constant_iterator<cudf::size_type>(1);
+  auto starts_itr = thrust::constant_iterator<cudf::size_type>(max_str_length / 3);
   auto stops_itr  = thrust::constant_iterator<cudf::size_type>(max_str_length / 2);
   cudf::test::fixed_width_column_wrapper<int32_t> starts(starts_itr, starts_itr + n_rows);
   cudf::test::fixed_width_column_wrapper<int32_t> stops(stops_itr, stops_itr + n_rows);
@@ -53,7 +53,9 @@ static void BM_slice(benchmark::State& state, slice_type rt)
   for (auto _ : state) {
     cuda_event_timer raii(state, true, cudf::get_default_stream());
     switch (rt) {
-      case position: cudf::strings::slice_strings(input, 1, max_str_length / 2); break;
+      case position:
+        cudf::strings::slice_strings(input, max_str_length / 3, max_str_length / 2);
+        break;
       case multi_position: cudf::strings::slice_strings(input, starts, stops); break;
       case delimiter: cudf::strings::slice_strings(input, std::string{" "}, 1); break;
       case multi_delimiter:
@@ -72,7 +74,7 @@ static void generate_bench_args(benchmark::internal::Benchmark* b)
   int const row_mult   = 8;
   int const min_rowlen = 1 << 5;
   int const max_rowlen = 1 << 13;
-  int const len_mult   = 4;
+  int const len_mult   = 2;
   generate_string_bench_args(b, min_rows, max_rows, row_mult, min_rowlen, max_rowlen, len_mult);
 }
 

cpp/include/cudf/strings/string_view.cuh

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019-2022, NVIDIA CORPORATION.
+ * Copyright (c) 2019-2023, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -137,7 +137,7 @@ __device__ inline string_view::const_iterator string_view::const_iterator::opera
 }
 
 __device__ inline string_view::const_iterator string_view::const_iterator::operator+(
-  string_view::const_iterator::difference_type offset)
+  string_view::const_iterator::difference_type offset) const
 {
   const_iterator tmp(*this);
   size_type adjust = abs(offset);
@@ -181,7 +181,7 @@ __device__ inline string_view::const_iterator& string_view::const_iterator::oper
 }
 
 __device__ inline string_view::const_iterator string_view::const_iterator::operator-(
-  string_view::const_iterator::difference_type offset)
+  string_view::const_iterator::difference_type offset) const
 {
   const_iterator tmp(*this);
   size_type adjust = abs(offset);
@@ -396,12 +396,12 @@ __device__ inline size_type string_view::rfind(char_utf8 chr, size_type pos, siz
 }
 
 // parameters are character position values
-__device__ inline string_view string_view::substr(size_type pos, size_type length) const
+__device__ inline string_view string_view::substr(size_type pos, size_type count) const
 {
-  size_type spos = byte_offset(pos);
-  size_type epos = byte_offset(pos + length);
-  if (epos > size_bytes()) epos = size_bytes();
-  if (spos >= epos) return string_view("", 0);
+  if (pos < 0 || pos >= length()) { return string_view{}; }
+  auto const itr  = begin() + pos;
+  auto const spos = itr.byte_offset();
+  auto const epos = count >= 0 ? (itr + count).byte_offset() : size_bytes();
   return string_view(data() + spos, epos - spos);
 }
 

cpp/include/cudf/strings/string_view.hpp

@@ -88,11 +88,11 @@ class string_view {
     __device__ inline const_iterator& operator++();
     __device__ inline const_iterator operator++(int);
     __device__ inline const_iterator& operator+=(difference_type);
-    __device__ inline const_iterator operator+(difference_type);
+    __device__ inline const_iterator operator+(difference_type) const;
     __device__ inline const_iterator& operator--();
     __device__ inline const_iterator operator--(int);
     __device__ inline const_iterator& operator-=(difference_type);
-    __device__ inline const_iterator operator-(difference_type);
+    __device__ inline const_iterator operator-(difference_type) const;
     __device__ inline bool operator==(const const_iterator&) const;
     __device__ inline bool operator!=(const const_iterator&) const;
     __device__ inline bool operator<(const const_iterator&) const;