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

[Nikita Popov via llvm-commits]

================
@@ -2779,6 +2781,60 @@ static bool hoistMulAddAssociation(Instruction &I, Loop &L,
   return true;
 }
 
+/// Reassociate general 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;
+
+  Instruction::BinaryOps Opcode = BO->getOpcode();
+  BinaryOperator *Op0 = dyn_cast<BinaryOperator>(BO->getOperand(0));
+
+  // Transform: "(LV op C1) op C2" ==> "LV op (C1 op C2)"
+  if (Op0 && Op0->getOpcode() == Opcode) {
+    Value *LV = Op0->getOperand(0);
+    Value *C1 = Op0->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?");
+    IRBuilder<> Builder(Preheader->getTerminator());
+    Value *Inv = Builder.CreateBinOp(Opcode, C1, C2, "invariant.op");
+
+    auto *NewBO =
+        BinaryOperator::Create(Opcode, LV, Inv, BO->getName() + ".reass", BO);
+    NewBO->copyIRFlags(BO);
----------------
nikic wrote:

This is incorrect. You can't preserve flags like this during a reassociation. It's possible to preserve flags in some cases, but this depends on the flags of *both* original instructions, the opcode and possibly also other operand characteristics like non-zero-ness or non-negative-ness.

https://github.com/llvm/llvm-project/pull/81608