Reland: [LoopUnroll] Make use of MaxTripCount for loops with pragma unroll (llvm#2294)

Author: ronlieb

llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp

@@ -797,7 +797,7 @@ static unsigned getFullUnrollBoostingFactor(const EstimatedUnrollCost &Cost,
 static std::optional<unsigned>
 shouldPragmaUnroll(Loop *L, const PragmaInfo &PInfo,
                    const unsigned TripMultiple, const unsigned TripCount,
-                   const UnrollCostEstimator UCE,
+                   unsigned MaxTripCount, const UnrollCostEstimator UCE,
                    const TargetTransformInfo::UnrollingPreferences &UP) {
 
   // Using unroll pragma
@@ -827,6 +827,10 @@ shouldPragmaUnroll(Loop *L, const PragmaInfo &PInfo,
     return TripCount;
   }
 
+  if (PInfo.PragmaEnableUnroll && !TripCount && MaxTripCount &&
+      MaxTripCount <= UnrollMaxUpperBound)
+    return MaxTripCount;
+
   // if didn't return until here, should continue to other priorties
   return std::nullopt;
 }
@@ -953,7 +957,7 @@ bool llvm::computeUnrollCount(
   // 1st priority is unroll count set by "unroll-count" option.
   // 2nd priority is unroll count set by pragma.
   if (auto UnrollFactor = shouldPragmaUnroll(L, PInfo, TripMultiple, TripCount,
-                                             UCE, UP)) {
+                                             MaxTripCount, UCE, UP)) {
     UP.Count = *UnrollFactor;
 
     if (UserUnrollCount || (PragmaCount > 0)) {