Get the instruction having loop id.

[hiraditya]

Summary

This is useful in moving the loop id from the instruction to another when a loop is rotated. See: D22630

[hiraditya]

Ping

[hiraditya]

Merged the changes with D22630

llvm/include/llvm/Analysis/LoopInfo.h

public:
   bool isAnnotatedParallel() const;
 
   /// Return the llvm.loop loop id metadata node for this loop if it is present.
   ///
   /// If this loop contains the same llvm.loop metadata on each branch to the
   /// header then the node is returned. If any latch instruction does not
   /// contain llvm.loop or or if multiple latches contain different nodes then
   /// 0 is returned.
-  MDNode *getLoopID() const;
+  std::pair<MDNode *, Instruction *> getLoopIDWithInstr() const;
+  MDNode *getLoopID() const { return getLoopIDWithInstr().first; }
+
   /// Set the llvm.loop loop id metadata for this loop.
   ///
   /// The LoopID metadata node will be added to each terminator instruction in
   /// the loop that branches to the loop header.
   ///
   /// The LoopID metadata node should have one or more operands and the first
   /// operand should should be the node itself.
   void setLoopID(MDNode *LoopID) const;

llvm/lib/Analysis/LoopInfo.cpp

for (BasicBlock *BB : this->blocks()) {
     for (Instruction &I : *BB)
       if (auto CS = CallSite(&I))
         if (CS.cannotDuplicate())
           return false;
   }
   return true;
 }
 
-MDNode *Loop::getLoopID() const {
+std::pair<MDNode *, Instruction *> Loop::getLoopIDWithInstr() const {
   MDNode *LoopID = nullptr;
+  Instruction *I = nullptr;
   if (isLoopSimplifyForm()) {
-    LoopID = getLoopLatch()->getTerminator()->getMetadata(LLVMContext::MD_loop);
+    I = getLoopLatch()->getTerminator();
+    LoopID = I->getMetadata(LLVMContext::MD_loop);
   } else {
     // Go through each predecessor of the loop header and check the
     // terminator for the metadata.
     BasicBlock *H = getHeader();
     for (BasicBlock *BB : predecessors(H)) {
       // Check if this terminator branches to the loop header.
       if (!this->contains(BB))
         continue;
 
       TerminatorInst *TI = BB->getTerminator();
       if (MDNode *MD = TI->getMetadata(LLVMContext::MD_loop)) {
-        if (!LoopID)
+        if (!LoopID) {
           LoopID = MD;
+          I = TI;
+        }
         else if (MD != LoopID) // Multiple MD_loop found => corrupt metadata.
-          return nullptr;
+          return { nullptr, nullptr };
       }
     }
   }
   if (!LoopID || LoopID->getNumOperands() == 0 ||
       LoopID->getOperand(0) != LoopID)
-    return nullptr;
-  return LoopID;
+    return { nullptr, nullptr };
+  return { LoopID, I };
 }
 
 void Loop::setLoopID(MDNode *LoopID) const {
   assert(LoopID && "Loop ID should not be null");
   assert(LoopID->getNumOperands() > 0 && "Loop ID needs at least one operand");
   assert(LoopID->getOperand(0) == LoopID && "Loop ID should refer to itself");
 
   if (isLoopSimplifyForm()) {