[llvm] f632c49 - Extract a helper function for computing estimate trip count of an exiting branch

[Philip Reames via llvm-commits]

Author: Philip Reames
Date: 2021-12-16T17:29:32-08:00
New Revision: f632c49478611c8d3eb777edd3302e16733fc7f5

URL: https://github.com/llvm/llvm-project/commit/f632c49478611c8d3eb777edd3302e16733fc7f5
DIFF: https://github.com/llvm/llvm-project/commit/f632c49478611c8d3eb777edd3302e16733fc7f5.diff

LOG: Extract a helper function for computing estimate trip count of an exiting branch

Plan to use this in following change to support estimated trip counts derived from multiple loop exits.

Added: 
    

Modified: 
    llvm/lib/Transforms/Utils/LoopUtils.cpp

Removed: 
    


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diff  --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
index d7877144ffd87..93157bd87c34e 100644
--- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -792,39 +792,51 @@ static BranchInst *getExpectedExitLoopLatchBranch(Loop *L) {
   return LatchBR;
 }
 
-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
-  BranchInst *LatchBranch = getExpectedExitLoopLatchBranch(L);
-  if (!LatchBranch)
-    return None;
-
+/// Return the estimated trip count for any exiting branch which dominates
+/// the loop latch.
+static Optional<uint64_t>
+getEstimatedTripCount(BranchInst *ExitingBranch, Loop *L,
+                      uint64_t &OrigExitWeight) {
   // To estimate the number of times the loop body was executed, we want to
   // know the number of times the backedge was taken, vs. the number of times
   // we exited the loop.
-  uint64_t BackedgeTakenWeight, LatchExitWeight;
-  if (!LatchBranch->extractProfMetadata(BackedgeTakenWeight, LatchExitWeight))
+  uint64_t LoopWeight, ExitWeight;
+  if (!ExitingBranch->extractProfMetadata(LoopWeight, ExitWeight))
     return None;
 
-  if (LatchBranch->getSuccessor(0) != L->getHeader())
-    std::swap(BackedgeTakenWeight, LatchExitWeight);
+  if (L->contains(ExitingBranch->getSuccessor(1)))
+    std::swap(LoopWeight, ExitWeight);
 
-  if (!LatchExitWeight)
+  if (!ExitWeight)
+    // Don't have a way to return predicated infinite
     return None;
 
-  if (EstimatedLoopInvocationWeight)
-    *EstimatedLoopInvocationWeight = LatchExitWeight;
+  OrigExitWeight = ExitWeight;
 
-  // Estimated backedge taken count is a ratio of the backedge taken weight by
-  // the weight of the edge exiting the loop, rounded to nearest.
-  uint64_t BackedgeTakenCount =
-      llvm::divideNearest(BackedgeTakenWeight, LatchExitWeight);
-  // Estimated trip count is one plus estimated backedge taken count.
-  return BackedgeTakenCount + 1;
+  // Estimated exit count is a ratio of the loop weight by the weight of the
+  // edge exiting the loop, rounded to nearest.
+  uint64_t ExitCount = llvm::divideNearest(LoopWeight, ExitWeight);
+  // Estimated trip count is one plus estimated exit count.
+  return ExitCount + 1;
+}
+
+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 (Optional<uint64_t> EstTripCount =
+        getEstimatedTripCount(LatchBranch, L, ExitWeight)) {
+      if (EstimatedLoopInvocationWeight)
+        *EstimatedLoopInvocationWeight = ExitWeight;
+      return *EstTripCount;
+    }
+  }
+  return None;
 }
 
 bool llvm::setLoopEstimatedTripCount(Loop *L, unsigned EstimatedTripCount,