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

[Nikita Popov via llvm-commits]

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@@ -2779,6 +2781,72 @@ 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) {
+  BinaryOperator *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;
+
+  BinaryOperator *BO0 = dyn_cast<BinaryOperator>(BO->getOperand(0));
+
+  // Transform: "(LV op C1) op C2" ==> "LV op (C1 op C2)"
+  if (BO0 && BO0->getOpcode() == Opcode && BO0->isAssociative()) {
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nikic wrote:

Early exit.

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