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

[Nikita Popov via llvm-commits]

================
@@ -50,6 +51,242 @@ 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 for canonicalizing to the form ~((Op1 | Op2)
+/// ^ Op0). More cases can be systematically added based on real-world
+/// justification for specific 3 input cases.
+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);
----------------
nikic wrote:

```suggestion
    Result = ConstantInt::getFalse(Op0->getType());
```
I don't think we need a helper function for this... (also for true below).

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