[llvm] [InstCombine] Improve bitfield addition (PR #77184)

[via llvm-commits]

================
@@ -3379,6 +3379,149 @@ Value *InstCombinerImpl::foldAndOrOfICmps(ICmpInst *LHS, ICmpInst *RHS,
   return foldAndOrOfICmpsUsingRanges(LHS, RHS, IsAnd);
 }
 
+struct BitFieldAddBitMask {
+  const APInt *Lower;
+  const APInt *Upper;
+};
+struct BitFieldOptBitMask {
+  const APInt *Lower;
+  const APInt *Upper;
+  const APInt *New;
+};
+struct BitFieldAddInfo {
+  Value *X;
+  Value *Y;
+  bool opt;
+  union {
+    BitFieldAddBitMask AddMask;
+    BitFieldOptBitMask OptMask;
+  };
+};
+
+static Value *foldBitFieldArithmetic(BinaryOperator &I,
+                                     InstCombiner::BuilderTy &Builder) {
+  auto *Disjoint = dyn_cast<PossiblyDisjointInst>(&I);
+  if (!Disjoint || !Disjoint->isDisjoint())
+    return nullptr;
+
+  unsigned BitWidth = I.getType()->getScalarSizeInBits();
+  auto AccumulateY = [&](Value *LoY, Value *UpY, APInt LoMask,
+                         APInt UpMask) -> Value * {
+    Value *Y = nullptr;
+    auto CLoY = dyn_cast_or_null<Constant>(LoY);
+    auto CUpY = dyn_cast_or_null<Constant>(UpY);
+    if ((CLoY == nullptr) ^ (CUpY == nullptr))
+      return nullptr;
+
+    if (CLoY && CUpY) {
+      APInt IUpY = CUpY->getUniqueInteger();
+      APInt ILoY = CLoY->getUniqueInteger();
+      if (!(IUpY.isSubsetOf(UpMask) && ILoY.isSubsetOf(LoMask)))
+        return nullptr;
+      Y = ConstantInt::get(CLoY->getType(), ILoY + IUpY);
+    } else if (LoY == UpY) {
+      Y = LoY;
+    }
+
+    return Y;
+  };
+
+  auto MatchBitFieldAdd =
+      [&](BinaryOperator &I) -> std::optional<BitFieldAddInfo> {
+    const APInt *OptLoMask, *OptUpMask, *LoMask, *UpMask, *UpMask2 = nullptr;
+    Value *X, *Y, *UpY;
+    auto BitFieldAddUpper = m_CombineOr(
+        m_And(m_c_Add(m_And(m_Value(X), m_APInt(UpMask)), m_Value(UpY)),
+              m_APInt(UpMask2)),
+        m_c_Add(m_And(m_Value(X), m_APInt(UpMask)), m_Value(UpY)));
+    auto BitFieldAdd =
+        m_c_Or(BitFieldAddUpper,
+               m_And(m_c_Add(m_Deferred(X), m_Value(Y)), m_APInt(LoMask)));
+    auto BitFieldAddIC =
+        m_c_Or(m_And(m_c_Add(m_Value(X), m_Value(Y)), m_APInt(LoMask)),
+               m_And(m_c_Add(m_Deferred(X), m_Value(UpY)), m_APInt(UpMask)));
+    auto OptBitFieldAdd = m_c_Or(
+        m_c_Xor(m_CombineOr(
+                    m_c_Add(m_And(m_Value(X), m_APInt(OptLoMask)),
+                            m_And(m_Value(Y), m_APInt(OptLoMask))),
+                    m_c_Add(m_And(m_Value(X), m_APInt(OptLoMask)), m_Value(Y))),
+                m_CombineOr(m_And(m_Deferred(X), m_APInt(OptUpMask)),
+                            m_And(m_c_Xor(m_Deferred(X), m_Value(UpY)),
+                                  m_APInt(OptUpMask)))),
+        BitFieldAddUpper);
+
+    if (match(&I, BitFieldAdd) || match(&I, BitFieldAddIC)) {
+      APInt Mask = APInt::getBitsSet(BitWidth, BitWidth - UpMask->countl_zero(),
+                                     BitWidth);
+      if (!((UpMask2 == nullptr || *UpMask == *UpMask2) &&
+            (LoMask->popcount() >= 2 && UpMask->popcount() >= 2) &&
+            (LoMask->isShiftedMask() && UpMask->isShiftedMask()) &&
+            ((*LoMask & *UpMask) == 0) &&
+            ((Mask ^ *LoMask ^ *UpMask).isAllOnes())))
+        return std::nullopt;
+
+      if (!(Y = AccumulateY(Y, UpY, *LoMask, *UpMask)))
+        return std::nullopt;
+
+      return {{X, Y, false, {{LoMask, UpMask}}}};
+    }
+
+    if (match(&I, OptBitFieldAdd)) {
+      APInt Mask = APInt::getBitsSet(
+          BitWidth, BitWidth - OptUpMask->countl_zero(), BitWidth);
+      APInt Mask2 = APInt::getBitsSet(
+          BitWidth, BitWidth - UpMask->countl_zero(), BitWidth);
+      if (!((UpMask2 == nullptr || *UpMask == *UpMask2) &&
+            (UpMask->isShiftedMask() && UpMask->popcount() >= 2) &&
+            ((*UpMask & (*OptLoMask | *OptUpMask)) == 0) &&
+            ((~*OptLoMask ^ Mask) == *OptUpMask) &&
+            (Mask2 ^ *UpMask ^ (*OptLoMask ^ *OptUpMask)).isAllOnes()))
+        return std::nullopt;
+
+      if (!(Y = AccumulateY(Y, UpY, (*OptLoMask + *OptUpMask), *UpMask)))
+        return std::nullopt;
+
+      struct BitFieldAddInfo Info = {X, Y, true, {{OptLoMask, OptUpMask}}};
+      Info.OptMask.New = UpMask;
+      return {Info};
+    }
+
+    return std::nullopt;
+  };
+
+  auto Info = MatchBitFieldAdd(I);
+  if (Info) {
+    Value *X = Info->X;
+    Value *Y = Info->Y;
+    APInt BitLoMask, BitUpMask;
+    if (Info->opt) {
+      unsigned NewHiBit = BitWidth - (Info->OptMask.New->countl_zero() + 1);
+      BitLoMask = *Info->OptMask.Lower | *Info->OptMask.New;
+      BitLoMask.clearBit(NewHiBit);
+      BitUpMask = *Info->OptMask.Upper;
+      BitUpMask.setBit(NewHiBit);
+    } else {
+      unsigned LowerHiBit = BitWidth - (Info->AddMask.Lower->countl_zero() + 1);
+      unsigned UpperHiBit = BitWidth - (Info->AddMask.Upper->countl_zero() + 1);
+      BitLoMask = *Info->AddMask.Lower | *Info->AddMask.Upper;
+      BitLoMask.clearBit(LowerHiBit);
+      BitLoMask.clearBit(UpperHiBit);
+      BitUpMask = APInt::getOneBitSet(BitWidth, LowerHiBit);
+      BitUpMask.setBit(UpperHiBit);
+    }
+
+    auto AndXLower = Builder.CreateAnd(X, BitLoMask);
+    auto AndYLower = Builder.CreateAnd(Y, BitLoMask);
+    auto Add = Builder.CreateNUWAdd(AndXLower, AndYLower);
+    auto Xor1 = Builder.CreateXor(X, Y);
+    auto AndUpper = Builder.CreateAnd(Xor1, BitUpMask);
+    auto Xor = Builder.CreateXor(Add, AndUpper);
+    return Xor;
+  }
+
+  return nullptr;
+}
----------------
goldsteinn wrote:

This entire impl is in desperate need of some comments.

https://github.com/llvm/llvm-project/pull/77184