[llvm] Reapply "[LICM] Fold associative binary ops to promote code hoisting (#81608)" (PR #100377)

[Sudharsan Veeravalli via llvm-commits]

================
@@ -2779,6 +2781,68 @@ static bool hoistMulAddAssociation(Instruction &I, Loop &L,
   return true;
 }
 
+/// Reassociate associative binary expressions of the form
+///
+/// 1. "(LV op C1) op C2" ==> "LV op (C1 op C2)"
+///
+/// where op is an associative binary op, LV is a loop variant, and C1 and C2
+/// are loop invariants that we want to hoist.
+///
+/// TODO: This can be extended to more cases such as
+/// 2. "C1 op (C2 op LV)" ==> "(C1 op C2) op LV"
+/// 3. "(C1 op LV) op C2" ==> "LV op (C1 op C2)" if op is commutative
+/// 4. "C1 op (LV op C2)" ==> "(C1 op C2) op LV" if op is commutative
+static bool hoistBOAssociation(Instruction &I, Loop &L,
+                               ICFLoopSafetyInfo &SafetyInfo,
+                               MemorySSAUpdater &MSSAU, AssumptionCache *AC,
+                               DominatorTree *DT) {
+  auto *BO = dyn_cast<BinaryOperator>(&I);
+  if (!BO || !BO->isAssociative())
+    return false;
+
+  // Only fold ADDs for now.
+  Instruction::BinaryOps Opcode = BO->getOpcode();
+  if (Opcode != Instruction::Add)
+    return false;
+
+  auto *BO0 = dyn_cast<BinaryOperator>(BO->getOperand(0));
+  if (!BO0 || BO0->getOpcode() != Opcode || !BO0->isAssociative())
+    return false;
+
+  // Transform: "(LV op C1) op C2" ==> "LV op (C1 op C2)"
+  Value *LV = BO0->getOperand(0);
+  Value *C1 = BO0->getOperand(1);
+  Value *C2 = BO->getOperand(1);
+
+  if (L.isLoopInvariant(LV) || !L.isLoopInvariant(C1) || !L.isLoopInvariant(C2))
+    return false;
+
+  auto *Preheader = L.getLoopPreheader();
+  assert(Preheader && "Loop is not in simplify form?");
+
+  auto *Inv = BinaryOperator::Create(Opcode, C1, C2, "invariant.op",
+                                     Preheader->getTerminator());
+  auto *NewBO =
+      BinaryOperator::Create(Opcode, LV, Inv, BO->getName() + ".reass", BO);
+
+  // Copy NUW for ADDs if both instructions have it.
+  if (Opcode == Instruction::Add && BO->hasNoUnsignedWrap() &&
+      BO0->hasNoUnsignedWrap()) {
+    Inv->setHasNoUnsignedWrap(true);
+    NewBO->setHasNoUnsignedWrap(true);
+  }
+
+  BO->replaceAllUsesWith(NewBO);
----------------
svs-quic wrote:

Hi, do we need to preserve the debug information of the original binary operator by copying it over to the new one?

Something like:
`NewBO->setDebugLoc(BO->getDebugLoc());`

https://github.com/llvm/llvm-project/pull/100377