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

[Mircea Trofin via llvm-commits]

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@@ -834,33 +865,86 @@ static std::optional<unsigned> getEstimatedTripCount(BranchInst *ExitingBranch,
     return std::numeric_limits<unsigned>::max();
 
   // Estimated trip count is one plus estimated exit count.
-  return ExitCount + 1;
+  uint64_t TC = ExitCount + 1;
+  LLVM_DEBUG(dbgs() << "estimateLoopTripCount: estimated trip count of " << TC
+                    << " for " << DbgLoop(L) << "\n");
+  return TC;
 }
 
-std::optional<unsigned>
-llvm::getLoopEstimatedTripCount(Loop *L,
-                                unsigned *EstimatedLoopInvocationWeight) {
-  // Currently we take the estimate exit count only from the loop latch,
-  // ignoring other exiting blocks.  This can overestimate the trip count
-  // if we exit through another exit, but can never underestimate it.
-  // TODO: incorporate information from other exits
-  if (BranchInst *LatchBranch = getExpectedExitLoopLatchBranch(L)) {
-    uint64_t ExitWeight;
-    if (std::optional<uint64_t> EstTripCount =
-            getEstimatedTripCount(LatchBranch, L, ExitWeight)) {
-      if (EstimatedLoopInvocationWeight)
-        *EstimatedLoopInvocationWeight = ExitWeight;
-      return *EstTripCount;
+std::optional<unsigned> llvm::getLoopEstimatedTripCount(
+    Loop *L, unsigned *EstimatedLoopInvocationWeight, bool DbgForInit) {
+  // If requested, either compute *EstimatedLoopInvocationWeight or return
+  // nullopt if cannot.
+  //
+  // TODO: Eventually, once all passes have migrated away from setting branch
+  // weights to indicate estimated trip counts, this function will drop the
+  // EstimatedLoopInvocationWeight parameter.
+  if (EstimatedLoopInvocationWeight) {
+    if (BranchInst *ExitingBranch = getExpectedExitLoopLatchBranch(L)) {
+      uint64_t LoopWeight, ExitWeight;
+      if (!extractBranchWeights(*ExitingBranch, LoopWeight, ExitWeight))
+        return std::nullopt;
+      if (L->contains(ExitingBranch->getSuccessor(1)))
+        std::swap(LoopWeight, ExitWeight);
+      if (!ExitWeight)
+        return std::nullopt;
+      *EstimatedLoopInvocationWeight = ExitWeight;
     }
   }
-  return std::nullopt;
+
+  // Return the estimated trip count from metadata unless the metadata is
+  // missing or has no value.
+  bool Missing;
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mtrofin wrote:

could you please assign it to a value (to avoid later accidental undefined values)

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