[llvm] fix for Issue #139786 - Missed Optimization: max(max(x, c1) << c2, c3) —> max(x << c2, c3) when c3 >= c1 * 2 ^ c2 (PR #140526)

[Yingwei Zheng via llvm-commits]

================
@@ -1171,6 +1171,84 @@ static Instruction *moveAddAfterMinMax(IntrinsicInst *II,
   return IsSigned ? BinaryOperator::CreateNSWAdd(NewMinMax, Add->getOperand(1))
                   : BinaryOperator::CreateNUWAdd(NewMinMax, Add->getOperand(1));
 }
+
+//Try canonicalize min/max(x << shamt, c<<shamt) into max(x, c) << shamt
+static Instruction *moveShiftAfterMinMax(IntrinsicInst *II, InstCombiner::BuilderTy &Builder) {
+  Intrinsic::ID MinMaxID = II->getIntrinsicID();
+  assert((MinMaxID == Intrinsic::smax || MinMaxID == Intrinsic::smin ||
+    MinMaxID == Intrinsic::umax || MinMaxID == Intrinsic::umin) &&
+   "Expected a min or max intrinsic");
+  
+  Value *Op0 = II->getArgOperand(0), *Op1 = II->getArgOperand(1);
+  Value *InnerMax;
+  const APInt *C;
+  if (!match(Op0, m_OneUse(m_BinOp(m_Value(InnerMax), m_APInt(C)))) || 
+      !match(Op1, m_APInt(C)))
+      return nullptr;
+  
+  auto* BinOpInst = cast<BinaryOperator>(Op0);
+  Instruction::BinaryOps BinOp = BinOpInst->getOpcode();
+  Value *X;
+  InnerMax = BinOpInst->getOperand(0);
+  // std::cout<< InnerMax->dump() <<std::endl;
+  if(!match(InnerMax,m_OneUse(m_Intrinsic<Intrinsic::umax>(m_Value(X),m_APInt(C))))){
+  if(!match(InnerMax,m_OneUse(m_Intrinsic<Intrinsic::smax>(m_Value(X),m_APInt(C))))){
+  if(!match(InnerMax,m_OneUse(m_Intrinsic<Intrinsic::umin>(m_Value(X),m_APInt(C))))){
+  if(!match(InnerMax,m_OneUse(m_Intrinsic<Intrinsic::smin>(m_Value(X),m_APInt(C))))){
+     return nullptr;
+  }}}}
+  
+  auto *InnerMaxInst = cast<IntrinsicInst>(InnerMax);
+
+  bool IsSigned = MinMaxID == Intrinsic::smax || MinMaxID == Intrinsic::smin;
+  if((IsSigned && !BinOpInst->hasNoSignedWrap()) ||
+     (!IsSigned && !BinOpInst->hasNoUnsignedWrap())) 
+     return nullptr;
+
+  // Check if BinOp is a left shift
+  if (BinOp != Instruction::Shl) {
+    return nullptr;
+  }
+
+  APInt C2=llvm::dyn_cast<llvm::ConstantInt>(BinOpInst->getOperand(1))->getValue() ;
+  APInt C3=llvm::dyn_cast<llvm::ConstantInt>(II->getArgOperand(1))->getValue();
+  APInt C1=llvm::dyn_cast<llvm::ConstantInt>(InnerMaxInst->getOperand(1))->getValue();
+
+  // Compute C1 * 2^C2
+  APInt Two = APInt(C2.getBitWidth(), 2);
+  APInt Pow2C2 = Two.shl(C2); // 2^C2
+  APInt C1TimesPow2C2 = C1 * Pow2C2; // C1 * 2^C2
+
+  // Check C3 >= C1 * 2^C2
+  if (C3.ult(C1TimesPow2C2)) {
+    return nullptr;
+  }
+
+  //Create new x binop c2
+  Value *NewBinOp = Builder.CreateBinOp(BinOp, InnerMaxInst->getOperand(0), BinOpInst->getOperand(1) );
+  
+  //Create new min/max intrinsic with new binop and c3
+  
+    if(IsSigned){
----------------
dtcxzyw wrote:

```suggestion
    if (auto *NewBinInst = dyn_cast<Instruction>(NewBinOp))
    if(IsSigned){
```
`NewBinOp` may be constant-folded.




https://github.com/llvm/llvm-project/pull/140526