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

[Simon Pilgrim via llvm-commits]

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@@ -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);
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RKSimon wrote:

use `auto *` (otherwise the static analysers will complain)

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