[llvm] [InstCombine] Offset both sides of an equality icmp (PR #134086)

[Nikita Popov via llvm-commits]

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@@ -5808,6 +5808,134 @@ static Instruction *foldICmpPow2Test(ICmpInst &I,
   return nullptr;
 }
 
+/// Find all possible pairs (BinOp, RHS) that BinOp V, RHS can be simplified.
+using OffsetOp = std::pair<Instruction::BinaryOps, Value *>;
+static void collectOffsetOp(Value *V, SmallVectorImpl<OffsetOp> &Offsets,
+                            bool AllowRecursion) {
+  Instruction *Inst = dyn_cast<Instruction>(V);
+  if (!Inst)
+    return;
+
+  switch (Inst->getOpcode()) {
+  case Instruction::Add: {
+    Constant *C;
+    if (match(Inst->getOperand(1), m_ImmConstant(C)) &&
+        !C->containsUndefOrPoisonElement()) {
+      if (Constant *NegC = ConstantExpr::getNeg(C))
+        Offsets.emplace_back(Instruction::Add, NegC);
+    }
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nikic wrote:

Would generating `sub, rhs` instead of specializing to constant RHS buy us anything?

https://github.com/llvm/llvm-project/pull/134086