[llvm] [PGO] Add `llvm.loop.estimated_trip_count` metadata (PR #148758)

[Mircea Trofin via llvm-commits]

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@@ -316,28 +318,73 @@ LLVM_ABI TransformationMode hasDistributeTransformation(const Loop *L);
 LLVM_ABI TransformationMode hasLICMVersioningTransformation(const Loop *L);
 /// @}
 
-/// Set input string into loop metadata by keeping other values intact.
-/// If the string is already in loop metadata update value if it is
-/// different.
-LLVM_ABI void addStringMetadataToLoop(Loop *TheLoop, const char *MDString,
-                                      unsigned V = 0);
-
-/// Returns a loop's estimated trip count based on branch weight metadata.
-/// In addition if \p EstimatedLoopInvocationWeight is not null it is
-/// initialized with weight of loop's latch leading to the exit.
-/// Returns a valid positive trip count, saturated at UINT_MAX, or std::nullopt
-/// when a meaningful estimate cannot be made.
+/// Set the string \p MDString into the loop metadata of \p TheLoop while
+/// keeping other loop metadata intact.  Set \p *V as its value, or set it
+/// without a value if \p V is \c std::nullopt to indicate the value is unknown.
+/// If \p MDString is already in the loop metadata, update it if its value (or
+/// lack of value) is different.  Return true if metadata was changed.
+LLVM_ABI bool addStringMetadataToLoop(Loop *TheLoop, const char *MDString,
+                                      std::optional<unsigned> V = 0);
+
+/// Return either:
+/// - The value of \c llvm.loop.estimated_trip_count from the loop metadata of
+///   \p L, if that metadata is present and has a value.
+/// - Else, a new estimate of the trip count from the latch branch weights of
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mtrofin wrote:

Ah, so the idea is to have the implementation here in this PR as a stop-gap, correct? So subsequent PRs would fix passes to write the metadata, and eventually `getLoopEstimatedTripCount` would just fetch metadata?

Maybe also add to the TODO in `getLoopEstimatedTripCount` that "...and rely solely on metadata" or smth?

https://github.com/llvm/llvm-project/pull/148758