[llvm] [InstCombine] Simplify and/or by replacing operands with constants (PR #77231)

[Yingwei Zheng via llvm-commits]

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@@ -2159,6 +2159,61 @@ Instruction *InstCombinerImpl::foldBinOpOfDisplacedShifts(BinaryOperator &I) {
   return BinaryOperator::Create(ShiftOp, NewC, ShAmt);
 }
 
+// Try to simplify X | Y by replacing occurrences of Y in X with 0.
+// Similarly, simplify X & Y by replacing occurrences of Y in X with -1.
+static Value *simplifyAndOrWithOpReplaced(Value *X, Value *Y, bool IsAnd,
+                                          InstCombinerImpl &IC,
+                                          unsigned Depth = 0) {
+  if (isa<Constant>(X) || X == Y)
+    return nullptr;
+
+  auto RecursivelyReplaceUses = [&](Instruction::BinaryOps Opcode, Value *Op0,
+                                    Value *Op1) -> Value * {
+    if (Depth == 2)
+      return nullptr;
+
+    // TODO: Relax the one-use constraint to clean up existing hard-coded
+    // simplifications.
+    if (!X->hasOneUse())
+      return nullptr;
+    Value *NewOp0 = simplifyAndOrWithOpReplaced(Op0, Y, IsAnd, IC, Depth + 1);
+    Value *NewOp1 = simplifyAndOrWithOpReplaced(Op1, Y, IsAnd, IC, Depth + 1);
+    if (!NewOp0 && !NewOp1)
+      return nullptr;
+    return IC.Builder.CreateBinOp(Opcode, NewOp0 ? NewOp0 : Op0,
+                                  NewOp1 ? NewOp1 : Op1);
+  };
+
+  Value *Op0, *Op1;
+  if (match(X, m_And(m_Value(Op0), m_Value(Op1)))) {
+    if (Op0 == Y || Op1 == Y)
+      return IsAnd ? (Op0 == Y ? Op1 : Op0)
+                   : Constant::getNullValue(X->getType());
----------------
dtcxzyw wrote:

> I will make it return the simplified result of X |/& Y.

Emm, it is hard to change the semantics of `simplifyAndOrWithOpReplaced` :(
I cannot guarantee that we never create more instructions than we save because we have to create two instructions in `RecursivelyReplaceUses` (one for `simplfiedOp0 bitop simplifiedOp1`, and the other for `simplifiedX and/or Y`).

I added some comments to clarify the semantics of `simplifyAndOrWithOpReplaced`.


https://github.com/llvm/llvm-project/pull/77231