[llvm] [LoopUnroll] Enhance the use of Optimization Remarks and `LLVM_DEBUG` (PR #178476)

[Justin Fargnoli via llvm-commits]

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@@ -807,38 +821,83 @@ static std::optional<unsigned>
 shouldPragmaUnroll(Loop *L, const PragmaInfo &PInfo,
                    const unsigned TripMultiple, const unsigned TripCount,
                    unsigned MaxTripCount, const UnrollCostEstimator UCE,
-                   const TargetTransformInfo::UnrollingPreferences &UP) {
+                   const TargetTransformInfo::UnrollingPreferences &UP,
+                   OptimizationRemarkEmitter *ORE) {
 
   // Using unroll pragma
   // 1st priority is unroll count set by "unroll-count" option.
 
   if (PInfo.UserUnrollCount) {
     if (UP.AllowRemainder &&
-        UCE.getUnrolledLoopSize(UP, (unsigned)UnrollCount) < UP.Threshold)
+        UCE.getUnrolledLoopSize(UP, (unsigned)UnrollCount) < UP.Threshold) {
+      LLVM_DEBUG(dbgs().indent(2) << "Unrolling with user-specified count: "
+                                  << UnrollCount << "\n");
       return (unsigned)UnrollCount;
+    }
+    LLVM_DEBUG(dbgs().indent(2)
+               << "Not unrolling with user count " << UnrollCount << ": "
+               << (!UP.AllowRemainder ? "remainder not allowed"
+                                      : "exceeds threshold")
+               << ".\n");
   }
 
   // 2nd priority is unroll count set by pragma.
   if (PInfo.PragmaCount > 0) {
-    if ((UP.AllowRemainder || (TripMultiple % PInfo.PragmaCount == 0)))
+    if ((UP.AllowRemainder || (TripMultiple % PInfo.PragmaCount == 0))) {
+      LLVM_DEBUG(dbgs().indent(2) << "Unrolling with pragma count: "
+                                  << PInfo.PragmaCount << "\n");
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justinfargnoli wrote:

Let's make all additional debug messages a separate PR. 

https://github.com/llvm/llvm-project/pull/178476