[llvm] [InstCombine] Canonicalize complex boolean expressions into ~((y | z) ^ x) via 3-input truth table (PR #149530)

[Yingwei Zheng via llvm-commits]

================
@@ -50,6 +51,205 @@ static Value *getFCmpValue(unsigned Code, Value *LHS, Value *RHS,
   return Builder.CreateFCmpFMF(NewPred, LHS, RHS, FMF);
 }
 
+/// This is to create optimal 3-variable boolean logic from truth tables.
+/// currently it supports the cases pertaining to the issue 97044. More cases
+/// can be added based on real-world justification for specific 3 input cases
+///  or with reviewer approval all 256 cases can be added (choose the
+///  canonicalizations found
+/// in x86InstCombine.cpp?)
+static Value *createLogicFromTable3Var(const std::bitset<8> &Table, Value *Op0,
+                                       Value *Op1, Value *Op2, Value *Root,
+                                       IRBuilderBase &Builder) {
+  uint8_t TruthValue = Table.to_ulong();
+  auto FoldConstant = [&](bool Val) {
+    Type *Ty = Op0->getType();
+    return Val ? ConstantInt::getTrue(Ty) : ConstantInt::getFalse(Ty);
+  };
+
+  Value *Result = nullptr;
+  switch (TruthValue) {
+  default:
+    return nullptr;
+  case 0x00: // Always FALSE
+    Result = FoldConstant(false);
+    break;
+  case 0xFF: // Always TRUE
+    Result = FoldConstant(true);
+    break;
+  case 0xE1: // ~((Op1 | Op2) ^ Op0)
+  {
+    Value *Or = Builder.CreateOr(Op1, Op2);
+    Value *Xor = Builder.CreateXor(Or, Op0);
+    Result = Builder.CreateNot(Xor);
+  } break;
+  case 0x60: // Op0 & (Op1 ^ Op2)
+  {
+    Value *Xor = Builder.CreateXor(Op1, Op2);
+    Result = Builder.CreateAnd(Op0, Xor);
+  } break;
+  case 0xD2: // ((Op1 | Op2) ^ Op0) ^ Op1
+  {
+    Value *Or = Builder.CreateOr(Op1, Op2);
+    Value *Xor1 = Builder.CreateXor(Or, Op0);
+    Result = Builder.CreateXor(Xor1, Op1);
+  } break;
+  }
+
+  return Result;
+}
+
+static std::tuple<Value *, Value *, Value *>
+extractThreeVariables(Value *Root) {
+  SmallPtrSet<Value *, 3> Variables;
+  SmallPtrSet<Value *, 32> Visited; // Prevent hanging during loop unrolling
+                                    // (see bitreverse-hang.ll)
+  SmallVector<Value *> Worklist;
+  Worklist.push_back(Root);
+
+  // Track all instructions to ensure they're in the same BB
+  BasicBlock *FirstBB = nullptr;
+
+  while (!Worklist.empty()) {
+    Value *V = Worklist.pop_back_val();
+
+    Value *NotV;
+    if (match(V, m_Not(m_Value(NotV)))) {
+      Visited.insert(NotV);
+      if (V == Root ||
+          V->hasOneUse()) { // Due to lack of cost-based heuristic, only
+                            // traverse if it belongs to this expression tree
+        Worklist.push_back(NotV);
+      }
+      continue;
+    }
+    if (auto *BO = dyn_cast<BinaryOperator>(V)) {
+      if (!BO->isBitwiseLogicOp())
+        return {nullptr, nullptr, nullptr};
+
+      // Check BB consistency
+      if (!FirstBB)
+        FirstBB = BO->getParent();
+      else if (BO->getParent() != FirstBB)
+        return {nullptr, nullptr, nullptr};
+
+      if (V == Root || V->hasOneUse()) {
+        Visited.insert(BO->getOperand(0));
+        Visited.insert(BO->getOperand(1));
+        Worklist.push_back(BO->getOperand(0));
+        Worklist.push_back(BO->getOperand(1));
+      }
+    } else if (isa<Argument>(V) || isa<Instruction>(V)) {
+      if (!isa<Constant>(V) && V != Root) {
+        Variables.insert(V);
+      }
+    }
+  }
+
+  if (Variables.size() == 3) {
+    // Sort variables by instruction order
+    SmallVector<Value *, 3> SortedVars(Variables.begin(), Variables.end());
+    llvm::sort(SortedVars, [](Value *A, Value *B) {
+      if (auto *IA = dyn_cast<Instruction>(A))
+        if (auto *IB = dyn_cast<Instruction>(B))
+          return IA->comesBefore(IB);
+      return A < B;
+    });
+    return {SortedVars[0], SortedVars[1], SortedVars[2]};
+  }
+  return {nullptr, nullptr, nullptr};
+}
+
+/// Evaluate a boolean expression with bit-vector inputs for all 8 combinations.
+static std::optional<std::bitset<8>>
+evaluateBooleanExpression(Value *Expr, Value *Op0, Value *Op1, Value *Op2) {
+  // Post-order traversal of the expression tree
----------------
dtcxzyw wrote:

Can we reuse the visited set in extractThreeVariables? `Instructions` should not contain nodes which are not belong to the tree (i.e., Op0-2 are the only leaf nodes).

https://github.com/llvm/llvm-project/pull/149530