[llvm] [IPO] Added attributor for identifying invariant loads (PR #141800)

[Shilei Tian via llvm-commits]

================
@@ -12534,6 +12535,301 @@ struct AAIndirectCallInfoCallSite : public AAIndirectCallInfo {
 };
 } // namespace
 
+/// --------------------- Invariant Load Pointer -------------------------------
+namespace {
+
+struct AAInvariantLoadPointerImpl
+    : public StateWrapper<BitIntegerState<uint8_t, 15>,
+                          AAInvariantLoadPointer> {
+
+  enum {
+    // pointer does not alias within the bounds of the function
+    IS_NOALIAS = 1 << 0,
+    // pointer is not involved in any effectful instructions within the bounds
+    // of the function
+    IS_NOEFFECT = 1 << 1,
+    // loads are invariant within the bounds of the function
+    IS_LOCALLY_INVARIANT = 1 << 2,
+    // memory lifetime is constrained within the bounds of the function
+    IS_LOCALLY_CONSTRAINED = 1 << 3,
+
+    IS_BEST_STATE = IS_NOALIAS | IS_NOEFFECT | IS_LOCALLY_INVARIANT |
+                    IS_LOCALLY_CONSTRAINED,
+  };
+  static_assert(getBestState() == IS_BEST_STATE, "Unexpected best state");
+
+  using Base =
+      StateWrapper<BitIntegerState<uint8_t, 15>, AAInvariantLoadPointer>;
+
+  // the BitIntegerState is optimistic about IS_NOALIAS and IS_NOEFFECT, but
+  // pessimistic about IS_KNOWN_INVARIANT
+  AAInvariantLoadPointerImpl(const IRPosition &IRP, Attributor &A)
+      : Base(IRP) {}
+
+  bool isKnownInvariant() const final {
+    return isKnownLocallyInvariant() && isKnown(IS_LOCALLY_CONSTRAINED);
+  }
+  bool isKnownLocallyInvariant() const final {
+    if (isKnown(IS_LOCALLY_INVARIANT))
+      return true;
+    return isKnown(IS_NOALIAS | IS_NOEFFECT);
+  }
+
+  bool isAssumedInvariant() const final {
+    return isAssumedLocallyInvariant() && isAssumed(IS_LOCALLY_CONSTRAINED);
+  }
+  bool isAssumedLocallyInvariant() const final {
+    if (isAssumed(IS_LOCALLY_INVARIANT))
+      return true;
+    return isAssumed(IS_NOALIAS | IS_NOEFFECT);
+  }
+
+  ChangeStatus updateImpl(Attributor &A) override {
+    if (isKnownInvariant())
+      return indicateOptimisticFixpoint();
+
+    ChangeStatus Changed = ChangeStatus::UNCHANGED;
+
+    Changed |= checkNoAlias(A);
+    if (requiresNoAlias() && !isAssumed(IS_NOALIAS))
+      return indicatePessimisticFixpoint();
+
+    Changed |= checkNoEffect(A);
+
+    // try to infer invariance from underlying objects
+    const auto *AUO = A.getOrCreateAAFor<AAUnderlyingObjects>(
+        getIRPosition(), this, DepClassTy::REQUIRED);
+
+    bool UsedAssumedInformation = false;
+    const auto IsInvariantLoadIfPointer = [&](const Value &V) {
+      if (!V.getType()->isPointerTy())
+        return true;
+      const auto *IsInvariantLoadPointer =
+          A.getOrCreateAAFor<AAInvariantLoadPointer>(IRPosition::value(V), this,
+                                                     DepClassTy::REQUIRED);
+      if (IsInvariantLoadPointer->isKnownLocallyInvariant())
+        return true;
+      if (!IsInvariantLoadPointer->isAssumedLocallyInvariant())
+        return false;
+
+      UsedAssumedInformation = true;
+      return true;
+    };
+    if (!AUO->forallUnderlyingObjects(IsInvariantLoadIfPointer))
+      return indicatePessimisticFixpoint();
+
+    if (!UsedAssumedInformation) {
+      // pointer is known (not assumed) to be locally invariant
+      addKnownBits(IS_LOCALLY_INVARIANT);
+      return Changed;
+    }
+
+    return Changed;
+  }
+
+  ChangeStatus manifest(Attributor &A) override {
+    if (!isKnownInvariant())
+      return ChangeStatus::UNCHANGED;
+
+    ChangeStatus Changed = ChangeStatus::UNCHANGED;
+    Value *Ptr = &getAssociatedValue();
+    const auto TagInvariantLoads = [&](const Use &U, bool &) {
+      if (U.get() != Ptr)
+        return true;
+      auto *I = dyn_cast<Instruction>(U.getUser());
+      if (!I)
+        return true;
+
+      // Ensure that we are only changing uses from the corresponding callgraph
+      // SSC in the case that the AA isn't run on the entire module
+      if (!A.isRunOn(I->getFunction()))
+        return true;
+
+      if (I->hasMetadata(LLVMContext::MD_invariant_load))
+        return true;
+
+      if (auto *LI = dyn_cast<LoadInst>(I)) {
+
+        LI->setMetadata(LLVMContext::MD_invariant_load,
+                        MDNode::get(LI->getContext(), {}));
+        Changed = ChangeStatus::CHANGED;
+      }
+      return true;
+    };
+
+    (void)A.checkForAllUses(TagInvariantLoads, *this, *Ptr);
+    return Changed;
+  }
+
+  /// See AbstractAttribute::getAsStr().
+  const std::string getAsStr(Attributor *) const override {
+    if (isKnownInvariant())
+      return "load-invariant pointer";
+    return "non-invariant pointer";
+  }
+
+  /// See AbstractAttribute::trackStatistics().
+  void trackStatistics() const override {}
+
+protected:
+  /// Indicate that invariance necessarily requires the pointer to be noalias.
+  virtual bool requiresNoAlias() const { return false; }
+
+private:
+  bool isExternal() const {
+    const auto *F = getAssociatedFunction();
+    if (!F)
+      return true;
+    return isCallableCC(F->getCallingConv()) &&
+           getPositionKind() != IRP_CALL_SITE_RETURNED;
+  }
+
+  ChangeStatus checkNoAlias(Attributor &A) {
+    if (isKnown(IS_NOALIAS) || !isAssumed(IS_NOALIAS))
+      return ChangeStatus::UNCHANGED;
+
+    // try to use AANoAlias
+    if (const auto *ANoAlias = A.getOrCreateAAFor<AANoAlias>(
+            getIRPosition(), this, DepClassTy::REQUIRED)) {
+      if (ANoAlias->isKnownNoAlias()) {
+        addKnownBits(IS_NOALIAS);
+        return ChangeStatus::UNCHANGED;
+      }
+
+      if (!ANoAlias->isAssumedNoAlias()) {
+        removeAssumedBits(IS_NOALIAS);
+        return ChangeStatus::CHANGED;
+      }
+
+      return ChangeStatus::UNCHANGED;
+    }
+
+    // try to infer noalias from argument attribute, since it is applicable for
+    // the duration of the function
+    if (const auto *Arg = getAssociatedArgument()) {
+      if (Arg->hasNoAliasAttr()) {
+        addKnownBits(IS_NOALIAS);
+        return ChangeStatus::UNCHANGED;
+      }
+
+      // noalias information is not provided, and cannot be inferred,
+      // so we conservatively assume the pointer aliases.
+      removeAssumedBits(IS_NOALIAS);
+      return ChangeStatus::CHANGED;
+    }
+
+    return ChangeStatus::UNCHANGED;
+  }
+
+  ChangeStatus checkNoEffect(Attributor &A) {
+    if (isKnown(IS_NOEFFECT) || !isAssumed(IS_NOEFFECT))
+      return ChangeStatus::UNCHANGED;
+
+    if (!getAssociatedFunction())
+      return indicatePessimisticFixpoint();
+
+    const auto HasNoEffectLoads = [&](const Use &U, bool &) {
+      if (const auto *LI = dyn_cast<LoadInst>(U.getUser()))
+        return !LI->mayHaveSideEffects();
+
+      return true;
----------------
shiltian wrote:

I'd combine them

https://github.com/llvm/llvm-project/pull/141800