[Polly][OpTree] Better report applied changes.

Print to dbgs() any taken action.

Also, read-only scalars do not require any action unless
-polly-analyze-read-only-scalars=true is used. Better refect this by
using ForwardingAction::triviallyForwardable and thus not bumping the
statistics.

polly/lib/Transform/ForwardOpTree.cpp

@@ -166,11 +166,14 @@ struct ForwardingAction {
   }
 
   /// Named ctor: The value can just be used without any preparation.
-  static ForwardingAction triviallyForwardable(bool IsProfitable) {
+  static ForwardingAction triviallyForwardable(bool IsProfitable, Value *Val) {
     ForwardingAction Result;
     Result.Decision =
         IsProfitable ? FD_CanForwardProfitably : FD_CanForwardLeaf;
-    Result.Execute = []() { return true; };
+    Result.Execute = [=]() {
+      LLVM_DEBUG(dbgs() << "    trivially forwarded: " << *Val << "\n");
+      return true;
+    };
     return Result;
   }
 
@@ -637,6 +640,8 @@ public:
         Access = TargetStmt->ensureValueRead(Inst);
       Access->setNewAccessRelation(SameVal);
 
+      LLVM_DEBUG(dbgs() << "    forwarded known content of " << *Inst
+                        << " which is " << SameVal << "\n");
       TotalReloads++;
       NumReloads++;
       return false;
@@ -703,6 +708,9 @@ public:
       // operands. This ensures that its operands are inserted before the
       // instruction using them.
       TargetStmt->prependInstruction(UseInst);
+
+      LLVM_DEBUG(dbgs() << "    forwarded speculable instruction: " << *UseInst
+                        << "\n");
       NumInstructionsCopied++;
       TotalInstructionsCopied++;
       return true;
@@ -736,7 +744,7 @@ public:
     case VirtualUse::Block:
     case VirtualUse::Hoisted:
       // These can be used anywhere without special considerations.
-      return ForwardingAction::triviallyForwardable(false);
+      return ForwardingAction::triviallyForwardable(false, UseVal);
 
     case VirtualUse::Synthesizable: {
       // Check if the value is synthesizable at the new location as well. This
@@ -752,7 +760,7 @@ public:
       VirtualUse TargetUse = VirtualUse::create(
           S, TargetStmt, TargetStmt->getSurroundingLoop(), UseVal, true);
       if (TargetUse.getKind() == VirtualUse::Synthesizable)
-        return ForwardingAction::triviallyForwardable(false);
+        return ForwardingAction::triviallyForwardable(false, UseVal);
 
       LLVM_DEBUG(
           dbgs() << "    Synthesizable would not be synthesizable anymore: "
@@ -761,11 +769,15 @@ public:
     }
 
     case VirtualUse::ReadOnly: {
+      if (!ModelReadOnlyScalars)
+        return ForwardingAction::triviallyForwardable(false, UseVal);
+
       // If we model read-only scalars, we need to create a MemoryAccess for it.
       auto ExecAction = [this, TargetStmt, UseVal]() {
-        if (ModelReadOnlyScalars)
         TargetStmt->ensureValueRead(UseVal);
 
+        LLVM_DEBUG(dbgs() << "    forwarded read-only value " << *UseVal
+                          << "\n");
         NumReadOnlyCopied++;
         TotalReadOnlyCopied++;
 

polly/test/ForwardOpTree/forward_readonly.ll

@@ -42,7 +42,8 @@ return:
 
 ; STATS: Statistics {
 ; STATS:     Instructions copied: 1
-; STATS:     Read-only accesses copied: 1
+; MODEL:     Read-only accesses copied: 1
+; NOMODEL:   Read-only accesses copied: 0
 ; STATS:     Operand trees forwarded: 1
 ; STATS:     Statements with forwarded operand trees: 1
 ; STATS: }