[RFC] IR: Define noalias.addrspace metadata

This is intended to solve a problem with lowering atomics in OpenMP and C++ common to AMDGPU and NVPTX. In OpenCL and CUDA, it is undefined behavior for an atomic instruction to modify an object in thread private memory. In OpenMP, it is defined. Correspondingly, the hardware does not handle this correctly. For AMDGPU, 32-bit atomics work and 64-bit atomics are silently dropped. We therefore need to codegen this by inserting a runtime address space check, performing the private case without atomics, and fallback to issuing the real atomic otherwise. This metadata allows us to avoid this extra check and branch. Handle this by introducing metadata intended to be applied to atomicrmw, indicating they cannot access the forbidden address space.
llvm · Aug 8, 2024 · e403d08 · e403d08
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diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
@@ -8015,6 +8015,42 @@ it will contain a list of ids, including the ids of the callsites in the
 full inline sequence, in order from the leaf-most call's id to the outermost
 inlined call.
 
+
+'``noalias.addrspace``' Metadata
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``noalias.addrspace`` metadata is used to identify memory
+operations which cannot access a range of address spaces. It is
+attached to memory instructions, including :ref:`atomicrmw
+<i_atomicrmw>`, :ref:`cmpxchg <i_cmpxchg>`, and :ref:`call <i_call>`
+instructions.
+
+This follows the same form as :ref:`range metadata <_range-metadata>`,
+except the field entries must be of type `i32`. The interpretation is
+the same numeric address spaces as applied to IR values.
+
+Example:
+
+.. code-block:: llvm
+    ; %ptr cannot point to an object allocated in addrspace(5)
+    %rmw.valid = atomicrmw and ptr %ptr, i64 %value seq_cst, !noalias.addrspace !0
+
+    ; Undefined behavior. The underlying object is allocated in one of the listed
+    ; address spaces.
+    %alloca = alloca i64, addrspace(5)
+    %alloca.cast = addrspacecast ptr addrspace(5) %alloca to ptr
+    %rmw.ub = atomicrmw and ptr %alloca.cast, i64 %value seq_cst, !noalias.addrspace !0
+
+    !0 = !{i32 5, i32 6}
+
+
+This is intended for use on targets with a notion of generic address
+spaces, which at runtime resolve to different physical memory
+spaces. The interpretation of the address space values is target
+specific. The behavior is undefined if the runtime memory address does
+resolve to an object defined in one of the indicated address spaces.
+
+
 Module Flags Metadata
 =====================
 

diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
@@ -53,6 +53,8 @@ Changes to the LLVM IR
 * The ``x86_mmx`` IR type has been removed. It will be translated to
   the standard vector type ``<1 x i64>`` in bitcode upgrade.
 
+* Introduced `noalias.addrspace` metadata.
+
 Changes to LLVM infrastructure
 ------------------------------
 

diff --git a/llvm/include/llvm/IR/FixedMetadataKinds.def b/llvm/include/llvm/IR/FixedMetadataKinds.def
@@ -52,3 +52,4 @@ LLVM_FIXED_MD_KIND(MD_pcsections, "pcsections", 37)
 LLVM_FIXED_MD_KIND(MD_DIAssignID, "DIAssignID", 38)
 LLVM_FIXED_MD_KIND(MD_coro_outside_frame, "coro.outside.frame", 39)
 LLVM_FIXED_MD_KIND(MD_mmra, "mmra", 40)
+LLVM_FIXED_MD_KIND(MD_noalias_addrspace, "noalias.addrspace", 41)
diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
@@ -515,8 +515,9 @@ class Verifier : public InstVisitor<Verifier>, VerifierSupport {
   void visitFunction(const Function &F);
   void visitBasicBlock(BasicBlock &BB);
   void verifyRangeMetadata(const Value &V, const MDNode *Range, Type *Ty,
-                           bool IsAbsoluteSymbol);
+                           bool IsAbsoluteSymbol, bool IsAddrSpaceRange);
   void visitRangeMetadata(Instruction &I, MDNode *Range, Type *Ty);
+  void visitNoaliasAddrspaceMetadata(Instruction &I, MDNode *Range, Type *Ty);
   void visitDereferenceableMetadata(Instruction &I, MDNode *MD);
   void visitProfMetadata(Instruction &I, MDNode *MD);
   void visitCallStackMetadata(MDNode *MD);
@@ -760,7 +761,7 @@ void Verifier::visitGlobalValue(const GlobalValue &GV) {
     if (const MDNode *AbsoluteSymbol =
             GO->getMetadata(LLVMContext::MD_absolute_symbol)) {
       verifyRangeMetadata(*GO, AbsoluteSymbol, DL.getIntPtrType(GO->getType()),
-                          true);
+                          true, false);
     }
   }
 
@@ -4128,7 +4129,8 @@ static bool isContiguous(const ConstantRange &A, const ConstantRange &B) {
 /// Verify !range and !absolute_symbol metadata. These have the same
 /// restrictions, except !absolute_symbol allows the full set.
 void Verifier::verifyRangeMetadata(const Value &I, const MDNode *Range,
-                                   Type *Ty, bool IsAbsoluteSymbol) {
+                                   Type *Ty, bool IsAbsoluteSymbol,
+                                   bool IsAddrSpaceRange) {
   unsigned NumOperands = Range->getNumOperands();
   Check(NumOperands % 2 == 0, "Unfinished range!", Range);
   unsigned NumRanges = NumOperands / 2;
@@ -4145,8 +4147,14 @@ void Verifier::verifyRangeMetadata(const Value &I, const MDNode *Range,
 
     Check(High->getType() == Low->getType(), "Range pair types must match!",
           &I);
-    Check(High->getType() == Ty->getScalarType(),
-          "Range types must match instruction type!", &I);
+
+    if (IsAddrSpaceRange) {
+      Check(High->getType()->isIntegerTy(32),
+            "noalias.addrspace type must be i32!", &I);
+    } else {
+      Check(High->getType() == Ty->getScalarType(),
+            "Range types must match instruction type!", &I);
+    }
 
     APInt HighV = High->getValue();
     APInt LowV = Low->getValue();
@@ -4185,7 +4193,14 @@ void Verifier::verifyRangeMetadata(const Value &I, const MDNode *Range,
 void Verifier::visitRangeMetadata(Instruction &I, MDNode *Range, Type *Ty) {
   assert(Range && Range == I.getMetadata(LLVMContext::MD_range) &&
          "precondition violation");
-  verifyRangeMetadata(I, Range, Ty, false);
+  verifyRangeMetadata(I, Range, Ty, false, false);
+}
+
+void Verifier::visitNoaliasAddrspaceMetadata(Instruction &I, MDNode *Range,
+                                             Type *Ty) {
+  assert(Range && Range == I.getMetadata(LLVMContext::MD_noalias_addrspace) &&
+         "precondition violation");
+  verifyRangeMetadata(I, Range, Ty, false, true);
 }
 
 void Verifier::checkAtomicMemAccessSize(Type *Ty, const Instruction *I) {
@@ -5177,6 +5192,13 @@ void Verifier::visitInstruction(Instruction &I) {
     visitRangeMetadata(I, Range, I.getType());
   }
 
+  if (MDNode *Range = I.getMetadata(LLVMContext::MD_noalias_addrspace)) {
+    Check(isa<LoadInst>(I) || isa<StoreInst>(I) || isa<AtomicRMWInst>(I) ||
+              isa<AtomicCmpXchgInst>(I) || isa<CallInst>(I),
+          "noalias.addrspace are only for memory operations!", &I);
+    visitNoaliasAddrspaceMetadata(I, Range, I.getType());
+  }
+
   if (I.hasMetadata(LLVMContext::MD_invariant_group)) {
     Check(isa<LoadInst>(I) || isa<StoreInst>(I),
           "invariant.group metadata is only for loads and stores", &I);

diff --git a/llvm/test/Assembler/noalias-addrspace-md.ll b/llvm/test/Assembler/noalias-addrspace-md.ll
@@ -0,0 +1,110 @@
+; RUN: llvm-as < %s | llvm-dis | FileCheck %s
+
+define i64 @atomicrmw_noalias_addrspace__0_1(ptr %ptr, i64 %val) {
+; CHECK-LABEL: define i64 @atomicrmw_noalias_addrspace__0_1(
+; CHECK-SAME: ptr [[PTR:%.*]], i64 [[VAL:%.*]]) {
+; CHECK-NEXT:    [[RET:%.*]] = atomicrmw add ptr [[PTR]], i64 [[VAL]] seq_cst, align 8, !noalias.addrspace [[META0:![0-9]+]]
+; CHECK-NEXT:    ret i64 [[RET]]
+;
+  %ret = atomicrmw add ptr %ptr, i64 %val seq_cst, align 8, !noalias.addrspace !0
+  ret i64 %ret
+}
+
+define i64 @atomicrmw_noalias_addrspace__0_2(ptr %ptr, i64 %val) {
+; CHECK-LABEL: define i64 @atomicrmw_noalias_addrspace__0_2(
+; CHECK-SAME: ptr [[PTR:%.*]], i64 [[VAL:%.*]]) {
+; CHECK-NEXT:    [[RET:%.*]] = atomicrmw add ptr [[PTR]], i64 [[VAL]] seq_cst, align 8, !noalias.addrspace [[META1:![0-9]+]]
+; CHECK-NEXT:    ret i64 [[RET]]
+;
+  %ret = atomicrmw add ptr %ptr, i64 %val seq_cst, align 8, !noalias.addrspace !1
+  ret i64 %ret
+}
+
+define i64 @atomicrmw_noalias_addrspace__1_3(ptr %ptr, i64 %val) {
+; CHECK-LABEL: define i64 @atomicrmw_noalias_addrspace__1_3(
+; CHECK-SAME: ptr [[PTR:%.*]], i64 [[VAL:%.*]]) {
+; CHECK-NEXT:    [[RET:%.*]] = atomicrmw add ptr [[PTR]], i64 [[VAL]] seq_cst, align 8, !noalias.addrspace [[META2:![0-9]+]]
+; CHECK-NEXT:    ret i64 [[RET]]
+;
+  %ret = atomicrmw add ptr %ptr, i64 %val seq_cst, align 8, !noalias.addrspace !2
+  ret i64 %ret
+}
+
+define i64 @atomicrmw_noalias_addrspace__multiple_ranges(ptr %ptr, i64 %val) {
+; CHECK-LABEL: define i64 @atomicrmw_noalias_addrspace__multiple_ranges(
+; CHECK-SAME: ptr [[PTR:%.*]], i64 [[VAL:%.*]]) {
+; CHECK-NEXT:    [[RET:%.*]] = atomicrmw add ptr [[PTR]], i64 [[VAL]] seq_cst, align 8, !noalias.addrspace [[META3:![0-9]+]]
+; CHECK-NEXT:    ret i64 [[RET]]
+;
+  %ret = atomicrmw add ptr %ptr, i64 %val seq_cst, align 8, !noalias.addrspace !3
+  ret i64 %ret
+}
+
+define i64 @load_noalias_addrspace__5_6(ptr %ptr) {
+; CHECK-LABEL: define i64 @load_noalias_addrspace__5_6(
+; CHECK-SAME: ptr [[PTR:%.*]]) {
+; CHECK-NEXT:    [[RET:%.*]] = load i64, ptr [[PTR]], align 4, !noalias.addrspace [[META4:![0-9]+]]
+; CHECK-NEXT:    ret i64 [[RET]]
+;
+  %ret = load i64, ptr %ptr, align 4, !noalias.addrspace !4
+  ret i64 %ret
+}
+
+define void @store_noalias_addrspace__5_6(ptr %ptr, i64 %val) {
+; CHECK-LABEL: define void @store_noalias_addrspace__5_6(
+; CHECK-SAME: ptr [[PTR:%.*]], i64 [[VAL:%.*]]) {
+; CHECK-NEXT:    store i64 [[VAL]], ptr [[PTR]], align 4, !noalias.addrspace [[META4]]
+; CHECK-NEXT:    ret void
+;
+  store i64 %val, ptr %ptr, align 4, !noalias.addrspace !4
+  ret void
+}
+
+define { i64, i1 } @cmpxchg_noalias_addrspace__5_6(ptr %ptr, i64 %val0, i64 %val1) {
+; CHECK-LABEL: define { i64, i1 } @cmpxchg_noalias_addrspace__5_6(
+; CHECK-SAME: ptr [[PTR:%.*]], i64 [[VAL0:%.*]], i64 [[VAL1:%.*]]) {
+; CHECK-NEXT:    [[RET:%.*]] = cmpxchg ptr [[PTR]], i64 [[VAL0]], i64 [[VAL1]] monotonic monotonic, align 8, !noalias.addrspace [[META4]]
+; CHECK-NEXT:    ret { i64, i1 } [[RET]]
+;
+  %ret = cmpxchg ptr %ptr, i64 %val0, i64 %val1 monotonic monotonic, align 8, !noalias.addrspace !4
+  ret { i64, i1 } %ret
+}
+
+declare void @foo()
+
+define void @call_noalias_addrspace__5_6(ptr %ptr) {
+; CHECK-LABEL: define void @call_noalias_addrspace__5_6(
+; CHECK-SAME: ptr [[PTR:%.*]]) {
+; CHECK-NEXT:    call void @foo(), !noalias.addrspace [[META4]]
+; CHECK-NEXT:    ret void
+;
+  call void @foo(), !noalias.addrspace !4
+  ret void
+}
+
+define void @call_memcpy_intrinsic_addrspace__5_6(ptr %dst, ptr %src, i64 %size) {
+; CHECK-LABEL: define void @call_memcpy_intrinsic_addrspace__5_6(
+; CHECK-SAME: ptr [[DST:%.*]], ptr [[SRC:%.*]], i64 [[SIZE:%.*]]) {
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr [[DST]], ptr [[SRC]], i64 [[SIZE]], i1 false), !noalias.addrspace [[META4]]
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.memcpy.p0.p0.i64(ptr %dst, ptr %src, i64 %size, i1 false), !noalias.addrspace !4
+  ret void
+}
+
+declare void @llvm.memcpy.p0.p0.i64(ptr noalias nocapture writeonly, ptr noalias nocapture readonly, i64, i1 immarg) #0
+
+attributes #0 = { nocallback nofree nounwind willreturn memory(argmem: readwrite) }
+
+!0 = !{i32 0, i32 1}
+!1 = !{i32 0, i32 2}
+!2 = !{i32 1, i32 3}
+!3 = !{i32 4, i32 6, i32 10, i32 55}
+!4 = !{i32 5, i32 6}
+;.
+; CHECK: [[META0]] = !{i32 0, i32 1}
+; CHECK: [[META1]] = !{i32 0, i32 2}
+; CHECK: [[META2]] = !{i32 1, i32 3}
+; CHECK: [[META3]] = !{i32 4, i32 6, i32 10, i32 55}
+; CHECK: [[META4]] = !{i32 5, i32 6}
+;.
diff --git a/llvm/test/Verifier/noalias-addrspace.ll b/llvm/test/Verifier/noalias-addrspace.ll
@@ -0,0 +1,60 @@
+; RUN: not llvm-as < %s -o /dev/null 2>&1 | FileCheck %s
+
+; CHECK: It should have at least one range!
+; CHECK-NEXT: !0 = !{}
+define i64 @noalias_addrspace__empty(ptr %ptr, i64 %val) {
+  %ret = atomicrmw add ptr %ptr, i64 %val seq_cst, !noalias.addrspace !0
+  ret i64 %ret
+}
+
+; CHECK: Unfinished range!
+; CHECK-NEXT: !1 = !{i32 0}
+define i64 @noalias_addrspace__single_field(ptr %ptr, i64 %val) {
+  %ret = atomicrmw add ptr %ptr, i64 %val seq_cst, !noalias.addrspace !1
+  ret i64 %ret
+}
+
+; CHECK: Range must not be empty!
+; CHECK-NEXT: !2 = !{i32 0, i32 0}
+define i64 @noalias_addrspace__0_0(ptr %ptr, i64 %val) {
+  %ret = atomicrmw add ptr %ptr, i64 %val seq_cst, !noalias.addrspace !2
+  ret i64 %ret
+}
+
+; CHECK: noalias.addrspace type must be i32!
+; CHECK-NEXT: %ret = atomicrmw add ptr %ptr, i64 %val seq_cst, align 8, !noalias.addrspace !3
+define i64 @noalias_addrspace__i64(ptr %ptr, i64 %val) {
+  %ret = atomicrmw add ptr %ptr, i64 %val seq_cst, !noalias.addrspace !3
+  ret i64 %ret
+}
+
+; CHECK: The lower limit must be an integer!
+define i64 @noalias_addrspace__fp(ptr %ptr, i64 %val) {
+  %ret = atomicrmw add ptr %ptr, i64 %val seq_cst, !noalias.addrspace !4
+  ret i64 %ret
+}
+
+; CHECK: The lower limit must be an integer!
+define i64 @noalias_addrspace__ptr(ptr %ptr, i64 %val) {
+  %ret = atomicrmw add ptr %ptr, i64 %val seq_cst, !noalias.addrspace !5
+  ret i64 %ret
+}
+
+; CHECK: The lower limit must be an integer!
+define i64 @noalias_addrspace__nonconstant(ptr %ptr, i64 %val) {
+  %ret = atomicrmw add ptr %ptr, i64 %val seq_cst, !noalias.addrspace !6
+  ret i64 %ret
+}
+
+@gv0 = global i32 0
+@gv1 = global i32 1
+
+!0 = !{}
+!1 = !{i32 0}
+!2 = !{i32 0, i32 0}
+!3 = !{i64 1, i64 5}
+!4 = !{float 0.0, float 2.0}
+!5 = !{ptr null, ptr addrspace(1) null}
+!6 = !{i32 ptrtoint (ptr @gv0 to i32), i32 ptrtoint (ptr @gv1 to i32) }
+
+