Introduce llvm.noalias.decl intrinsic

The ``llvm.experimental.noalias.scope.decl`` intrinsic identifies where a noalias
scope is declared. When the intrinsic is duplicated, a decision must
also be made about the scope: depending on the reason of the duplication,
the scope might need to be duplicated as well.

Reviewed By: nikic, jdoerfert

Differential Revision: https://reviews.llvm.org/D93039

docs/LangRef.rst

@@ -19603,6 +19603,82 @@ Semantics:
 This function returns the same values as the libm ``trunc`` functions
 would and handles error conditions in the same way.
 
+.. _int_experimental_noalias_scope_decl:
+
+'``llvm.experimental.noalias.scope.decl``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+
+::
+
+      declare void @llvm.experimental.noalias.scope.decl(metadata !id.scope.list)
+
+Overview:
+"""""""""
+
+The ``llvm.experimental.noalias.scope.decl`` intrinsic identifies where a
+noalias scope is declared. When the intrinsic is duplicated, a decision must
+also be made about the scope: depending on the reason of the duplication,
+the scope might need to be duplicated as well.
+
+
+Arguments:
+""""""""""
+
+The ``!id.scope.list`` argument is metadata that is a list of ``noalias``
+metadata references. The format is identical to that required for ``noalias``
+metadata. This list must have exactly one element.
+
+Semantics:
+""""""""""
+
+The ``llvm.experimental.noalias.scope.decl`` intrinsic identifies where a
+noalias scope is declared. When the intrinsic is duplicated, a decision must
+also be made about the scope: depending on the reason of the duplication,
+the scope might need to be duplicated as well.
+
+For example, when the intrinsic is used inside a loop body, and that loop is
+unrolled, the associated noalias scope must also be duplicated. Otherwise, the
+noalias property it signifies would spill across loop iterations, whereas it
+was only valid within a single iteration.
+
+.. code-block:: llvm
+
+  ; This examples shows two possible positions for noalias.decl and how they impact the semantics:
+  ; If it is outside the loop (Version 1), then %a and %b are noalias across *all* iterations.
+  ; If it is inside the loop (Version 2), then %a and %b are noalias only within *one* iteration.
+  declare void @decl_in_loop(i8* %a.base, i8* %b.base) {
+  entry:
+    ; call void @llvm.experimental.noalias.scope.decl(metadata !2) ; Version 1: noalias decl outside loop
+    br label %loop
+  
+  loop:
+    %a = phi i8* [ %a.base, %entry ], [ %a.inc, %loop ]
+    %b = phi i8* [ %b.base, %entry ], [ %b.inc, %loop ]
+    ; call void @llvm.experimental.noalias.scope.decl(metadata !2) ; Version 2: noalias decl inside loop
+    %val = load i8, i8* %a, !alias.scope !2
+    store i8 %val, i8* %b, !noalias !2
+    %a.inc = getelementptr inbounds i8, i8* %a, i64 1
+    %b.inc = getelementptr inbounds i8, i8* %b, i64 1
+    %cond = call i1 @cond()
+    br i1 %cond, label %loop, label %exit
+  
+  exit:
+    ret void
+  }
+  
+  !0 = !{!0} ; domain
+  !1 = !{!1, !0} ; scope
+  !2 = !{!1} ; scope list
+
+Multiple calls to `@llvm.experimental.noalias.scope.decl` for the same scope
+are possible, but one should never dominate another. Violations are pointed out
+by the verifier as they indicate a problem in either a transformation pass or
+the input.
+
 
 Floating Point Environment Manipulation intrinsics
 --------------------------------------------------

include/llvm/IR/IRBuilder.h

@@ -858,6 +858,13 @@ public:
   CallInst *CreateAssumption(Value *Cond,
                              ArrayRef<OperandBundleDef> OpBundles = llvm::None);
 
+  /// Create a llvm.experimental.noalias.scope.decl intrinsic call.
+  Instruction *CreateNoAliasScopeDeclaration(Value *Scope);
+  Instruction *CreateNoAliasScopeDeclaration(MDNode *ScopeTag) {
+    return CreateNoAliasScopeDeclaration(
+        MetadataAsValue::get(Context, ScopeTag));
+  }
+
   /// Create a call to the experimental.gc.statepoint intrinsic to
   /// start a new statepoint sequence.
   CallInst *CreateGCStatepointCall(uint64_t ID, uint32_t NumPatchBytes,

include/llvm/IR/Intrinsics.h

@@ -34,6 +34,9 @@ class AttributeList;
 /// function known by LLVM. The enum values are returned by
 /// Function::getIntrinsicID().
 namespace Intrinsic {
+  // Abstraction for the arguments of the noalias intrinsics
+  static const int NoAliasScopeDeclScopeArg = 0;
+
   // Intrinsic ID type. This is an opaque typedef to facilitate splitting up
   // the enum into target-specific enums.
   typedef unsigned ID;

include/llvm/IR/Intrinsics.td

@@ -539,6 +539,16 @@ def int_readcyclecounter : DefaultAttrsIntrinsic<[llvm_i64_ty]>;
 def int_assume        : DefaultAttrsIntrinsic<[], [llvm_i1_ty], [IntrWillReturn,
                                                      NoUndef<ArgIndex<0>>]>;
 
+// 'llvm.experimental.noalias.scope.decl' intrinsic: Inserted at the location of
+// noalias scope declaration. Makes it possible to identify that a noalias scope
+// is only valid inside the body of a loop.
+//
+// Purpose of the different arguments:
+// - arg0: id.scope: metadata representing the scope declaration.
+def int_experimental_noalias_scope_decl
+    : DefaultAttrsIntrinsic<[], [llvm_metadata_ty],
+        [IntrInaccessibleMemOnly]>; // blocks LICM and some more
+
 // Stack Protector Intrinsic - The stackprotector intrinsic writes the stack
 // guard to the correct place on the stack frame.
 def int_stackprotector : DefaultAttrsIntrinsic<[], [llvm_ptr_ty, llvm_ptrptr_ty], []>;

lib/CodeGen/IntrinsicLowering.cpp

@@ -329,6 +329,7 @@ void IntrinsicLowering::LowerIntrinsicCall(CallInst *CI) {
     break;
 
   case Intrinsic::assume:
+  case Intrinsic::experimental_noalias_scope_decl:
   case Intrinsic::var_annotation:
     break;   // Strip out these intrinsics
 

lib/CodeGen/SelectionDAG/FastISel.cpp

@@ -1252,6 +1252,8 @@ bool FastISel::selectIntrinsicCall(const IntrinsicInst *II) {
   case Intrinsic::sideeffect:
   // Neither does the assume intrinsic; it's also OK not to codegen its operand.
   case Intrinsic::assume:
+  // Neither does the llvm.experimental.noalias.scope.decl intrinsic
+  case Intrinsic::experimental_noalias_scope_decl:
     return true;
   case Intrinsic::dbg_declare: {
     const DbgDeclareInst *DI = cast<DbgDeclareInst>(II);

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -6466,10 +6466,13 @@ void SelectionDAGBuilder::visitIntrinsicCall(const CallInst &I,
     // Drop the intrinsic, but forward the value
     setValue(&I, getValue(I.getOperand(0)));
     return;
+
   case Intrinsic::assume:
+  case Intrinsic::experimental_noalias_scope_decl:
   case Intrinsic::var_annotation:
   case Intrinsic::sideeffect:
-    // Discard annotate attributes, assumptions, and artificial side-effects.
+    // Discard annotate attributes, noalias scope declarations, assumptions, and
+    // artificial side-effects.
     return;
 
   case Intrinsic::codeview_annotation: {

lib/IR/IRBuilder.cpp

@@ -452,6 +452,13 @@ IRBuilderBase::CreateAssumption(Value *Cond,
   return createCallHelper(FnAssume, Ops, this, "", nullptr, OpBundles);
 }
 
+Instruction *IRBuilderBase::CreateNoAliasScopeDeclaration(Value *Scope) {
+  Module *M = BB->getModule();
+  auto *FnIntrinsic = Intrinsic::getDeclaration(
+      M, Intrinsic::experimental_noalias_scope_decl, {});
+  return createCallHelper(FnIntrinsic, {Scope}, this);
+}
+
 /// Create a call to a Masked Load intrinsic.
 /// \p Ptr       - base pointer for the load
 /// \p Alignment - alignment of the source location

lib/IR/Verifier.cpp

@@ -115,6 +115,11 @@
 
 using namespace llvm;
 
+static cl::opt<bool> VerifyNoAliasScopeDomination(
+    "verify-noalias-scope-decl-dom", cl::Hidden, cl::init(false),
+    cl::desc("Ensure that llvm.experimental.noalias.scope.decl for identical "
+             "scopes are not dominating"));
+
 namespace llvm {
 
 struct VerifierSupport {
@@ -313,6 +318,8 @@ class Verifier : public InstVisitor<Verifier>, VerifierSupport {
 
   TBAAVerifier TBAAVerifyHelper;
 
+  SmallVector<IntrinsicInst *, 4> NoAliasScopeDecls;
+
   void checkAtomicMemAccessSize(Type *Ty, const Instruction *I);
 
 public:
@@ -360,6 +367,8 @@ public:
     LandingPadResultTy = nullptr;
     SawFrameEscape = false;
     SiblingFuncletInfo.clear();
+    verifyNoAliasScopeDecl();
+    NoAliasScopeDecls.clear();
 
     return !Broken;
   }
@@ -536,6 +545,9 @@ private:
 
   /// Verify all-or-nothing property of DIFile source attribute within a CU.
   void verifySourceDebugInfo(const DICompileUnit &U, const DIFile &F);
+
+  /// Verify the llvm.experimental.noalias.scope.decl declarations
+  void verifyNoAliasScopeDecl();
 };
 
 } // end anonymous namespace
@@ -5163,6 +5175,10 @@ void Verifier::visitIntrinsicCall(Intrinsic::ID ID, CallBase &Call) {
            &Call);
     break;
   }
+  case Intrinsic::experimental_noalias_scope_decl: {
+    NoAliasScopeDecls.push_back(cast<IntrinsicInst>(&Call));
+    break;
+  }
   };
 }
 
@@ -5513,6 +5529,76 @@ void Verifier::verifySourceDebugInfo(const DICompileUnit &U, const DIFile &F) {
            "inconsistent use of embedded source");
 }
 
+void Verifier::verifyNoAliasScopeDecl() {
+  if (NoAliasScopeDecls.empty())
+    return;
+
+  // only a single scope must be declared at a time.
+  for (auto *II : NoAliasScopeDecls) {
+    assert(II->getIntrinsicID() == Intrinsic::experimental_noalias_scope_decl &&
+           "Not a llvm.experimental.noalias.scope.decl ?");
+    const auto *ScopeListMV = dyn_cast<MetadataAsValue>(
+        II->getOperand(Intrinsic::NoAliasScopeDeclScopeArg));
+    Assert(ScopeListMV != nullptr,
+           "llvm.experimental.noalias.scope.decl must have a MetadataAsValue "
+           "argument",
+           II);
+
+    const auto *ScopeListMD = dyn_cast<MDNode>(ScopeListMV->getMetadata());
+    Assert(ScopeListMD != nullptr, "!id.scope.list must point to an MDNode",
+           II);
+    Assert(ScopeListMD->getNumOperands() == 1,
+           "!id.scope.list must point to a list with a single scope", II);
+  }
+
+  // Only check the domination rule when requested. Once all passes have been
+  // adapted this option can go away.
+  if (!VerifyNoAliasScopeDomination)
+    return;
+
+  // Now sort the intrinsics based on the scope MDNode so that declarations of
+  // the same scopes are next to each other.
+  auto GetScope = [](IntrinsicInst *II) {
+    const auto *ScopeListMV = cast<MetadataAsValue>(
+        II->getOperand(Intrinsic::NoAliasScopeDeclScopeArg));
+    return &cast<MDNode>(ScopeListMV->getMetadata())->getOperand(0);
+  };
+
+  // We are sorting on MDNode pointers here. For valid input IR this is ok.
+  // TODO: Sort on Metadata ID to avoid non-deterministic error messages.
+  auto Compare = [GetScope](IntrinsicInst *Lhs, IntrinsicInst *Rhs) {
+    return GetScope(Lhs) < GetScope(Rhs);
+  };
+
+  llvm::sort(NoAliasScopeDecls, Compare);
+
+  // Go over the intrinsics and check that for the same scope, they are not
+  // dominating each other.
+  auto ItCurrent = NoAliasScopeDecls.begin();
+  while (ItCurrent != NoAliasScopeDecls.end()) {
+    auto CurScope = GetScope(*ItCurrent);
+    auto ItNext = ItCurrent;
+    do {
+      ++ItNext;
+    } while (ItNext != NoAliasScopeDecls.end() &&
+             GetScope(*ItNext) == CurScope);
+
+    // [ItCurrent, ItNext[ represents the declarations for the same scope.
+    // Ensure they are not dominating each other
+    for (auto *I : llvm::make_range(ItCurrent, ItNext)) {
+      for (auto *J : llvm::make_range(ItCurrent, ItNext)) {
+        if (I != J) {
+          Assert(!DT.dominates(I, J),
+                 "llvm.experimental.noalias.scope.decl dominates another one "
+                 "with the same scope",
+                 I);
+        }
+      }
+    }
+    ItCurrent = ItNext;
+  }
+}
+
 //===----------------------------------------------------------------------===//
 //  Implement the public interfaces to this file...
 //===----------------------------------------------------------------------===//

test/Verifier/noalias_scope_decl.ll

@@ -0,0 +1,61 @@
+; RUN: not llvm-as -disable-output --verify-noalias-scope-decl-dom < %s 2>&1 | FileCheck %s
+
+define void @test_single_scope01() nounwind ssp {
+  tail call void @llvm.experimental.noalias.scope.decl(metadata !2)
+  ret void
+}
+
+define void @test_single_scope02() nounwind ssp {
+  tail call void @llvm.experimental.noalias.scope.decl(metadata !5)
+  ret void
+}
+; CHECK: !id.scope.list must point to a list with a single scope
+; CHECK-NEXT:   tail call void @llvm.experimental.noalias.scope.decl(metadata !5)
+
+define void @test_single_scope03() nounwind ssp {
+  tail call void @llvm.experimental.noalias.scope.decl(metadata !"test")
+  ret void
+}
+; CHECK-NEXT: !id.scope.list must point to an MDNode
+; CHECK-NEXT:   tail call void @llvm.experimental.noalias.scope.decl(metadata !"test")
+
+define void @test_dom01() nounwind ssp {
+  tail call void @llvm.experimental.noalias.scope.decl(metadata !2)
+  tail call void @llvm.experimental.noalias.scope.decl(metadata !8)
+  ret void
+}
+
+define void @test_dom02() nounwind ssp {
+  tail call void @llvm.experimental.noalias.scope.decl(metadata !2)
+  tail call void @llvm.experimental.noalias.scope.decl(metadata !6)
+  ret void
+}
+; CHECK-NEXT: llvm.experimental.noalias.scope.decl dominates another one with the same scope
+; CHECK-NEXT:   tail call void @llvm.experimental.noalias.scope.decl(metadata !2)
+
+define void @test_dom03() nounwind ssp {
+  tail call void @llvm.experimental.noalias.scope.decl(metadata !2)
+  tail call void @llvm.experimental.noalias.scope.decl(metadata !2)
+  ret void
+}
+; CHECK-NEXT: llvm.experimental.noalias.scope.decl dominates another one with the same scope
+; CHECK-NEXT:   tail call void @llvm.experimental.noalias.scope.decl(metadata !2)
+
+; CHECK-NOT: llvm.experimental.noalias.scope.decl
+
+; Function Attrs: inaccessiblememonly nounwind
+declare void @llvm.experimental.noalias.scope.decl(metadata) #1
+
+attributes #1 = { inaccessiblememonly nounwind }
+!llvm.module.flags = !{!0}
+!llvm.ident = !{!1}
+
+!0 = !{i32 1, !"wchar_size", i32 4}
+!1 = !{!"clang"}
+!2 = !{!3}
+!3 = distinct !{!3, !4, !"test: pA"}
+!4 = distinct !{!4, !"test"}
+!5 = !{!3, !3}
+!6 = !{!3}
+!7 = distinct !{!7, !4, !"test: pB"}
+!8 = !{!7}