[llvm] [LoopRotation] Enable LoopRotation with -Oz if header folds (PR #72842)

[Nikita Popov via llvm-commits]

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@@ -365,6 +365,65 @@ static void updateBranchWeights(BranchInst &PreHeaderBI, BranchInst &LoopBI,
   }
 }
 
+// Check if duplicating the header to the preheader will fold all duplicated
+// instructions, terminating with an unconditional jump. So No code duplication
+// would be necessary.
+static bool canRotateWithoutDuplication(Loop *L, llvm::LoopInfo *LI,
+                                        llvm::SimplifyQuery SQ) {
+  BasicBlock *OrigHeader = L->getHeader();
+  BasicBlock *OrigPreheader = L->getLoopPreheader();
+
+  BranchInst *T = dyn_cast<BranchInst>(OrigHeader->getTerminator());
+  assert(T && T->isConditional() && "Header Terminator should be conditional!");
+
+  // a value map to holds the values incoming from preheader
+  ValueToValueMapTy ValueMap;
+
+  // iterate over non-debug instructions and check which ones fold
+  for (Instruction &Inst : OrigHeader->instructionsWithoutDebug()) {
+    // For PHI nodes, the values in the cloned header are the values in the
+    // preheader
+    if (PHINode *PN = dyn_cast<PHINode>(&Inst)) {
+      InsertNewValueIntoMap(ValueMap, PN,
+                            PN->getIncomingValueForBlock(OrigPreheader));
+      continue;
+    }
+    if (Inst.mayHaveSideEffects())
+      return false;
+
+    Instruction *C = Inst.clone();
+    C->insertBefore(&Inst);
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nikic wrote:

Why is it necessary to actually clone and remap instructions for this? Can you simplifyInstructionWithOperands() instead?

https://github.com/llvm/llvm-project/pull/72842