Add reverse(ContainerTy) range adapter.

For cases where we needed a foreach loop in reverse over a container,
we had to do something like

  for (const GlobalValue *GV : make_range(TypeInfos.rbegin(),
                                          TypeInfos.rend())) {

This provides a convenience method which shortens this to

  for (const GlobalValue *GV : reverse(TypeInfos)) {

There are 2 versions of this, with a preference to the rbegin() version.

The first uses rbegin() and rend() to construct an iterator_range.

The second constructs an iterator_range from the begin() and end() methods
wrapped in std::reverse_iterator's.

Reviewed by David Blaikie.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243563 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/STLExtras.h

@@ -196,6 +196,30 @@ inline mapped_iterator<ItTy, FuncTy> map_iterator(const ItTy &I, FuncTy F) {
   return mapped_iterator<ItTy, FuncTy>(I, F);
 }
 
+// Returns an iterator_range over the given container which iterates in reverse.
+// Note that the container must have rbegin()/rend() methods for this to work.
+template<typename ContainerTy>
+auto reverse(ContainerTy &C)->decltype(make_range(C.rbegin(), C.rend())) {
+  return make_range(C.rbegin(), C.rend());
+}
+
+// Returns a std::reverse_iterator wrapped around the given iterator.
+template<typename IteratorTy>
+std::reverse_iterator<IteratorTy> make_reverse_iterator(IteratorTy It) {
+  return std::reverse_iterator<IteratorTy>(It);
+}
+
+// Returns an iterator_range over the given container which iterates in reverse.
+// Note that the container must have begin()/end() methods which return
+// bidirectional iterators for this to work.
+template<typename ContainerTy>
+auto reverse(ContainerTy &&C)
+  ->decltype(make_range(make_reverse_iterator(std::end(C)),
+                        make_reverse_iterator(std::begin(C)))) {
+  return make_range(make_reverse_iterator(std::end(C)),
+                    make_reverse_iterator(std::begin(C)));
+}
+
 //===----------------------------------------------------------------------===//
 //     Extra additions to <utility>
 //===----------------------------------------------------------------------===//

lib/CodeGen/AsmPrinter/ARMException.cpp

@@ -115,8 +115,7 @@ void ARMException::emitTypeInfos(unsigned TTypeEncoding) {
     Entry = TypeInfos.size();
   }
 
-  for (const GlobalValue *GV : make_range(TypeInfos.rbegin(),
-                                          TypeInfos.rend())) {
+  for (const GlobalValue *GV : reverse(TypeInfos)) {
     if (VerboseAsm)
       Asm->OutStreamer->AddComment("TypeInfo " + Twine(Entry--));
     Asm->EmitTTypeReference(GV, TTypeEncoding);

unittests/ADT/CMakeLists.txt

@@ -28,6 +28,7 @@ set(ADTSources
   PointerIntPairTest.cpp
   PointerUnionTest.cpp
   PostOrderIteratorTest.cpp
+  RangeAdapterTest.cpp
   SCCIteratorTest.cpp
   SmallPtrSetTest.cpp
   SmallStringTest.cpp

unittests/ADT/RangeAdapterTest.cpp

@@ -0,0 +1,77 @@
+//===- RangeAdapterTest.cpp - Unit tests for range adapters  --------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/iterator_range.h"
+#include "llvm/ADT/STLExtras.h"
+#include "gtest/gtest.h"
+
+#include <iterator>
+#include <list>
+#include <vector>
+
+using namespace llvm;
+
+namespace {
+
+// A wrapper around vector which exposes rbegin(), rend().
+class ReverseOnlyVector {
+  std::vector<int> Vec;
+public:
+  ReverseOnlyVector(std::initializer_list<int> list) : Vec(list) { }
+
+  typedef std::vector<int>::reverse_iterator reverse_iterator;
+  reverse_iterator rbegin() { return Vec.rbegin(); }
+  reverse_iterator rend() { return Vec.rend(); }
+};
+
+// A wrapper around vector which exposes begin(), end(), rbegin() and rend().
+// begin() and end() don't have implementations as this ensures that we will
+// get a linker error if reverse() chooses begin()/end() over rbegin(), rend().
+class BidirectionalVector {
+  std::vector<int> Vec;
+
+public:
+  BidirectionalVector(std::initializer_list<int> list) : Vec(list) { }
+
+  typedef std::vector<int>::iterator iterator;
+  iterator begin();
+  iterator end();
+
+  typedef std::vector<int>::reverse_iterator reverse_iterator;
+  reverse_iterator rbegin() { return Vec.rbegin(); }
+  reverse_iterator rend() { return Vec.rend(); }
+};
+
+// Test fixture
+template <typename T>
+class RangeAdapterTest : public ::testing::Test { };
+
+typedef ::testing::Types<std::vector<int>,
+                         std::list<int>,
+                         int[4],
+                         ReverseOnlyVector,
+                         BidirectionalVector,
+                         const std::vector<int>,
+                         const std::list<int>,
+                         const int[4]> RangeAdapterTestTypes;
+TYPED_TEST_CASE(RangeAdapterTest, RangeAdapterTestTypes);
+
+TYPED_TEST(RangeAdapterTest, TrivialOperation) {
+  TypeParam v = { 0, 1, 2, 3 };
+
+  int counter = 3;
+  for (int i : reverse(v))
+    EXPECT_EQ(i, counter--);
+
+  counter = 0;
+  for (int i : reverse(reverse(v)))
+    EXPECT_EQ(i, counter++);
+}
+
+} // anonymous namespace