[mlir] Add a utility iterator range that repeats a given value `n` times.

[rriddle]

Summary

This range is useful when an desired API expects a range or when comparing two different ranges for equality, but the underlying data is a splat. This range removes the need to explicitly construct a vector in those cases.

Depends On D74682

[mehdi_amini]

Can we add this to llvm STLExtras? (since we should upstream all these utilities anyway...)

mlir/include/mlir/Support/STLExtras.h

358

You're the native speaker, but I would have expected to read make_repeated_range?

[rriddle]

In D74683#1884106, @mehdi_amini wrote:

Can we add this to llvm STLExtras? (since we should upstream all these utilities anyway...)

I was going to originally, but this relies on indexed_accessor_range which also needs to be upstreamed.

[mehdi_amini]

In D74683#1884111, @rriddle wrote:

In D74683#1884106, @mehdi_amini wrote:

Can we add this to llvm STLExtras? (since we should upstream all these utilities anyway...)

I was going to originally, but this relies on indexed_accessor_range which also needs to be upstreamed.

OK, but that's a good reminder that we need to upstream all this :)

[rriddle]

Resolve comments.

mlir/include/mlir/Support/STLExtras.h

 template <typename ContainerTy> auto make_second_range(ContainerTy &&c) {
   return llvm::map_range(
       std::forward<ContainerTy>(c),
       [](decltype((*std::begin(c))) elt) -> decltype((elt.second)) {
         return elt.second;
       });
 }
 
+/// A range class that repeats a specific value for a set number of times.
+template <typename T>
+class RepeatRange
+    : public detail::indexed_accessor_range_base<RepeatRange<T>, T, const T> {
+public:
+  using detail::indexed_accessor_range_base<
+      RepeatRange<T>, T, const T>::indexed_accessor_range_base;
+
+  /// Given that we are repeating a specific value, we can simply return that
+  /// value when offsetting the base or dereferencing the iterator.
+  static T offset_base(const T &val, ptrdiff_t) { return val; }
+  static const T &dereference_iterator(const T &val, ptrdiff_t) { return val; }
+};
+
+/// Make a range that repeats the given value 'n' times.
+template <typename ValueTy>
+RepeatRange<ValueTy> make_repeated_range(const ValueTy &value, size_t n) {
+  return RepeatRange<ValueTy>(value, n);
+}
+
 /// Returns true of the given range only contains a single element.
 template <typename ContainerTy> bool has_single_element(ContainerTy &&c) {
   auto it = std::begin(c), e = std::end(c);
   return it != e && std::next(it) == e;
 }
 
 //===----------------------------------------------------------------------===//
 //     Extra additions to <type_traits>