[llvm] [LLVM][ADT] Add helper class for working with caches (PR #171008)

[Jakub Kuderski via llvm-commits]

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@@ -2610,6 +2610,30 @@ template <typename T> using has_sizeof = decltype(sizeof(T));
 template <typename T>
 constexpr bool is_incomplete_v = !is_detected<detail::has_sizeof, T>::value;
 
+/// A utility for working with maps that allows concisely expressing "perform
+/// and cache this expensive computation only if it isn't already cached".
+/// Use it like so:
+///
+/// ```cpp
+/// std::unordered_map<K, V> Cache;
+/// auto& Value = Cache.try_emplace(
+///     Key, llvm::defer {[] { /* heavy work */ }}).first->second;
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kuhar wrote:

I don't get it -- how is this different than passing the lambda directly?

https://github.com/llvm/llvm-project/pull/171008