[LoopUtils] Don't wrap in getLoopEstimatedTripCount (#129080)

getLoopEstimatedTripCount returns the trip count based on profiling
data, and its documentation says that it could return 0 when the trip
count is zero, but this is not the case: a valid trip count can never be
zero, and it returns 0 when the unsigned ExitCount is incremented by 1
and wraps. Some callers are careful about checking for this zero value
in an std::optional, but it makes for an API with footguns, as a
std::optional return value indicates that a non-nullopt value would be a
valid trip count. Fix this by explicitly returning std::nullopt when the
return value would wrap, and strip additional checks in callers. This
also fixes a minor bug in LoopVectorize.

1 files changed, 11 insertions, 13 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopPeel.cpp b/llvm/lib/Transforms/Utils/LoopPeel.cpp
index 9a24c1b..0f3a92b 100644
--- a/llvm/lib/Transforms/Utils/LoopPeel.cpp
+++ b/llvm/lib/Transforms/Utils/LoopPeel.cpp
@@ -640,20 +640,18 @@ void llvm::computePeelCount(Loop *L, unsigned LoopSize,
     LLVM_DEBUG(dbgs() << "Profile-based estimated trip count is "
                       << *EstimatedTripCount << "\n");
 
-    if (*EstimatedTripCount) {
-      if (*EstimatedTripCount + AlreadyPeeled <= MaxPeelCount) {
-        unsigned PeelCount = *EstimatedTripCount;
-        LLVM_DEBUG(dbgs() << "Peeling first " << PeelCount << " iterations.\n");
-        PP.PeelCount = PeelCount;
-        return;
-      }
-      LLVM_DEBUG(dbgs() << "Already peel count: " << AlreadyPeeled << "\n");
-      LLVM_DEBUG(dbgs() << "Max peel count: " << UnrollPeelMaxCount << "\n");
-      LLVM_DEBUG(dbgs() << "Loop cost: " << LoopSize << "\n");
-      LLVM_DEBUG(dbgs() << "Max peel cost: " << Threshold << "\n");
-      LLVM_DEBUG(dbgs() << "Max peel count by cost: "
-                        << (Threshold / LoopSize - 1) << "\n");
+    if (*EstimatedTripCount + AlreadyPeeled <= MaxPeelCount) {
+      unsigned PeelCount = *EstimatedTripCount;
+      LLVM_DEBUG(dbgs() << "Peeling first " << PeelCount << " iterations.\n");
+      PP.PeelCount = PeelCount;
+      return;
     }
+    LLVM_DEBUG(dbgs() << "Already peel count: " << AlreadyPeeled << "\n");
+    LLVM_DEBUG(dbgs() << "Max peel count: " << UnrollPeelMaxCount << "\n");
+    LLVM_DEBUG(dbgs() << "Loop cost: " << LoopSize << "\n");
+    LLVM_DEBUG(dbgs() << "Max peel cost: " << Threshold << "\n");
+    LLVM_DEBUG(dbgs() << "Max peel count by cost: "
+                      << (Threshold / LoopSize - 1) << "\n");
   }
 }