[llvm] Attributor: Add noalias.addrspace attribute for store and load (PR #136553)

[Shilei Tian via llvm-commits]

================
@@ -12784,6 +12785,172 @@ struct AAAddressSpaceCallSiteArgument final : AAAddressSpaceImpl {
 };
 } // namespace
 
+/// ------------------------ No Alias Address Space  ---------------------------
+namespace {
+struct AANoAliasAddrSpaceImpl : public AANoAliasAddrSpace {
+  AANoAliasAddrSpaceImpl(const IRPosition &IRP, Attributor &A)
+      : AANoAliasAddrSpace(IRP, A) {}
+
+  void initialize(Attributor &A) override {
+    assert(getAssociatedType()->isPtrOrPtrVectorTy() &&
+           "Associated value is not a pointer");
+
+    if (!A.getInfoCache().getFlatAddressSpace().has_value()) {
+      indicatePessimisticFixpoint();
+      return;
+    }
+
+    unsigned FlatAS = A.getInfoCache().getFlatAddressSpace().value();
+    unsigned AS = getAssociatedType()->getPointerAddressSpace();
+    if (AS != FlatAS) {
+      removeAssumedBits(1 << AS);
+      indicateOptimisticFixpoint();
+    }
+  }
+
+  ChangeStatus updateImpl(Attributor &A) override {
+    unsigned FlatAS = A.getInfoCache().getFlatAddressSpace().value();
+    uint32_t OrigAssumed = getAssumed();
+
+    auto CheckAddressSpace = [&](Value &Obj) {
+      if (isa<UndefValue>(&Obj))
+        return true;
+      // Handle argument in flat address space only has addrspace cast uses
+      if (auto *Arg = dyn_cast<Argument>(&Obj)) {
+        if (Arg->getType()->getPointerAddressSpace() == FlatAS) {
+          for (auto *U : Arg->users()) {
+            auto *ASCI = dyn_cast<AddrSpaceCastInst>(U);
+            if (!ASCI)
+              return false;
+            if (ASCI->getDestAddressSpace() == FlatAS)
+              return false;
+            removeAssumedBits(1 << ASCI->getDestAddressSpace());
+          }
+        }
+      }
+      removeAssumedBits(1 << Obj.getType()->getPointerAddressSpace());
+      return true;
+    };
+
+    auto *AUO = A.getOrCreateAAFor<AAUnderlyingObjects>(getIRPosition(), this,
+                                                        DepClassTy::REQUIRED);
+    if (!AUO->forallUnderlyingObjects(CheckAddressSpace))
+      return indicatePessimisticFixpoint();
+
+    return OrigAssumed == getAssumed() ? ChangeStatus::UNCHANGED
+                                       : ChangeStatus::CHANGED;
+  }
+
+  /// See AbstractAttribute::manifest(...).
+  ChangeStatus manifest(Attributor &A) override {
+    if (!A.getInfoCache().getFlatAddressSpace().has_value())
+      return ChangeStatus::UNCHANGED;
+
+    unsigned FlatAS = A.getInfoCache().getFlatAddressSpace().value();
+    unsigned AS = getAssociatedType()->getPointerAddressSpace();
+    if (AS != FlatAS)
+      return ChangeStatus::UNCHANGED;
----------------
shiltian wrote:

As a generic AA, I agree that the meaning of each address space doesn't really matter. However, I think the tricky part here is that on some targets (like GPUs), AS0 aliases with others by default. So even if we explicitly say noalias for AS3, if we don't also say noalias for AS0, we don't actually get a strong guarantee that AS3 isn't aliased. That would make the use of `noalias.addrspace` ill-formed in this context.

https://github.com/llvm/llvm-project/pull/136553