[llvm] [InstCombine] Fold ((X << nuw Z) binop nuw Y) >>u Z --> X binop nuw (Y >>u Z) (PR #88193)

[Matt Arsenault via llvm-commits]

================
@@ -1259,6 +1259,52 @@ Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
       match(Op1, m_SpecificIntAllowPoison(BitWidth - 1)))
     return new ZExtInst(Builder.CreateIsNotNeg(X, "isnotneg"), Ty);
 
+  Value *Y;
+  if (match(Op0, m_OneUse(m_NUWSub(m_NUWShl(m_Value(X), m_Specific(Op1)),
+                                   m_Value(Y))))) {
+    Value *NewLshr = Builder.CreateLShr(Y, Op1, "", I.isExact());
+    auto *NewSub = BinaryOperator::CreateNUWSub(NewLshr, X);
+    NewSub->setHasNoSignedWrap(
+        cast<OverflowingBinaryOperator>(Op0)->hasNoSignedWrap());
+    return NewSub;
+  }
+
+  auto isSuitableBinOpcode = [](Instruction::BinaryOps BinOpcode) {
+    switch (BinOpcode) {
+    default:
+      return false;
+    case Instruction::Add:
+    case Instruction::Or:
+    case Instruction::Xor:
+      // And does not work here, and sub is handled separately.
+      return true;
+    }
+  };
+
+  // If both the add and the shift are nuw, then:
+  // ((X << nuw Z) binop nuw Y) >>u Z --> X binop nuw (Y >>u Z)
+  if (match(Op0, m_OneUse(m_c_BinOp(m_NUWShl(m_Value(X), m_Specific(Op1)),
+                                    m_Value(Y))))) {
+    BinaryOperator *Op0OB = cast<BinaryOperator>(Op0);
+    bool canFold = false;
+    if (isSuitableBinOpcode(Op0OB->getOpcode())) {
+      if (auto *Inst = dyn_cast<OverflowingBinaryOperator>(Op0)) {
+        if (Inst->hasNoUnsignedWrap())
+          canFold = true;
+      } else
+        canFold = true;
+
+      if (canFold) {
----------------
arsenm wrote:

You can avoid the canFold variable: 

if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(Op0); 
    !OBO || OBO->hasNoUnsignedWrap()) {

https://github.com/llvm/llvm-project/pull/88193