Only guard loop metadata that has non-debug info in it

Author: dominik-steenken

llvm/include/llvm/IR/Instruction.h

@@ -367,6 +367,10 @@ class Instruction : public User,
   /// Return true if this instruction has any metadata attached to it.
   bool hasMetadata() const { return DbgLoc || Value::hasMetadata(); }
 
+  // Return true if this instruction contains loop metadata other than
+  // a debug location
+  bool hasLoopMetadataOtherThanDebugLoc() const;
+
   /// Return true if this instruction has metadata attached to it other than a
   /// debug location.
   bool hasMetadataOtherThanDebugLoc() const { return Value::hasMetadata(); }

llvm/lib/IR/Instruction.cpp

@@ -12,13 +12,15 @@
 
 #include "llvm/IR/Instruction.h"
 #include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/IR/AttributeMask.h"
 #include "llvm/IR/Attributes.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Operator.h"
@@ -461,6 +463,29 @@ bool Instruction::hasPoisonGeneratingMetadata() const {
          hasMetadata(LLVMContext::MD_align);
 }
 
+bool Instruction::hasLoopMetadataOtherThanDebugLoc() const {
+  // If there is no loop metadata at all, we also don't have
+  // non-debug loop metadata, obviously.
+  if (!hasMetadata(LLVMContext::MD_loop))
+    return false;
+
+  // If we do have loop metadata, retrieve it.
+  MDNode *LoopMD = getMetadata(LLVMContext::MD_loop);
+
+  // Check if the existing operands are debug locations. This loop
+  // should terminate after at most three iterations. Skip
+  // the first item because it is a self-reference.
+  for (const MDOperand &Op : llvm::drop_begin(LoopMD->operands())) {
+    // check for debug location type by attempting a cast.
+    if (!dyn_cast<DILocation>(Op)) {
+      return true;
+    }
+  }
+
+  // If we get here, then all we have is debug locations in the loop metadata.
+  return false;
+}
+
 void Instruction::dropPoisonGeneratingMetadata() {
   eraseMetadata(LLVMContext::MD_range);
   eraseMetadata(LLVMContext::MD_nonnull);

llvm/lib/Transforms/Utils/Local.cpp

@@ -1279,7 +1279,7 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
   // |    for.body <---- (md2)
   // |_______|  |______|
   if (Instruction *TI = BB->getTerminator())
-    if (TI->hasMetadata(LLVMContext::MD_loop))
+    if (TI->hasLoopMetadataOtherThanDebugLoc())
       for (BasicBlock *Pred : predecessors(BB))
         if (Instruction *PredTI = Pred->getTerminator())
           if (PredTI->hasMetadata(LLVMContext::MD_loop))