[Flang][OpenMP] Defer descriptor mapping for assumed dummy argument types

This PR adds deferral of descriptor maps until they are neccessary for assumed
dummy argument types. The intent is to avoid a problem where a user can
inadvertently map a temporary local descriptor to device without their knowledge and
proceed to never unmap it. This temporary local descriptor remains lodged in OpenMP
device memory and the next time another variable or descriptor residing in the same
stack address is mapped we incur a runtime OpenMP map error as we try to remap the
same address.

This fix was discussed with the OpenMP committee and applies to OpenMP 5.2 and
below, future versions of OpenMP can avoid this issue via the attatch semantics
added to the specification.

Author: agozillon

flang/lib/Optimizer/OpenMP/MapInfoFinalization.cpp

@@ -75,6 +75,44 @@ class MapInfoFinalizationPass
   ///      |                  |
   std::map<mlir::Operation *, mlir::Value> localBoxAllocas;
 
+  // List of deferrable descriptors to process at the end of
+  // the pass.
+  llvm::SmallVector<mlir::Operation *> deferrableDesc;
+
+  // Check if the declaration operation we have refers to a dummy
+  // function argument.
+  bool isDummyArgument(mlir::Value mappedValue) {
+    if (auto declareOp = mlir::dyn_cast_if_present<hlfir::DeclareOp>(
+            mappedValue.getDefiningOp()))
+      if (auto dummyScope = declareOp.getDummyScope())
+        return true;
+    return false;
+  }
+
+  // Relevant for OpenMP < 5.2, where attach semantics and rules don't exist.
+  // As descriptors were an unspoken implementation detail in these versions
+  // there's certain cases where the user (and the compiler implementation)
+  // can create data mapping errors by having temporary descriptors stuck
+  // in memory. The main example is calling an 'target enter data map'
+  // without a corresponding exit on an assumed shape or size dummy
+  // argument, a local stack descriptor is generated, gets mapped and
+  // is then left on device. A user doesn't realize what they've done as
+  // the OpenMP specification isn't explicit on descriptor handling in
+  // earlier versions and as far as Fortran is concerned this si something
+  // hidden from a user. To avoid this we can defer the descriptor mapping
+  // in these cases until target or target data regions, when we can be
+  // sure they have a clear limited scope on device.
+  bool canDeferDescriptorMapping(mlir::Value descriptor) {
+    if (fir::isAllocatableType(descriptor.getType()) ||
+        fir::isPointerType(descriptor.getType()))
+      return false;
+    if (isDummyArgument(descriptor) &&
+        (fir::isAssumedType(descriptor.getType()) ||
+         fir::isAssumedShape(descriptor.getType())))
+      return true;
+    return false;
+  }
+
   /// getMemberUserList gathers all users of a particular MapInfoOp that are
   /// other MapInfoOp's and places them into the mapMemberUsers list, which
   /// records the map that the current argument MapInfoOp "op" is part of
@@ -126,13 +164,16 @@ class MapInfoFinalizationPass
   /// fir::BoxOffsetOp we utilise to access the descriptor datas
   /// base address can be utilised.
   mlir::Value getDescriptorFromBoxMap(mlir::omp::MapInfoOp boxMap,
-                                      fir::FirOpBuilder &builder) {
+                                      fir::FirOpBuilder &builder,
+                                      bool &canDescBeDeferred) {
     mlir::Value descriptor = boxMap.getVarPtr();
     if (!fir::isTypeWithDescriptor(boxMap.getVarType()))
       if (auto addrOp = mlir::dyn_cast_if_present<fir::BoxAddrOp>(
               boxMap.getVarPtr().getDefiningOp()))
         descriptor = addrOp.getVal();
 
+    canDescBeDeferred = canDeferDescriptorMapping(descriptor);
+
     if (!mlir::isa<fir::BaseBoxType>(descriptor.getType()) &&
         !fir::factory::isOptionalArgument(descriptor.getDefiningOp()))
       return descriptor;
@@ -283,8 +324,7 @@ class MapInfoFinalizationPass
 
   /// Check if the mapOp is present in the HasDeviceAddr clause on
   /// the userOp. Only applies to TargetOp.
-  bool isHasDeviceAddr(mlir::omp::MapInfoOp mapOp, mlir::Operation *userOp) {
-    assert(userOp && "Expecting non-null argument");
+  bool isHasDeviceAddr(mlir::omp::MapInfoOp mapOp, mlir::Operation &userOp) {
     if (auto targetOp = llvm::dyn_cast<mlir::omp::TargetOp>(userOp)) {
       for (mlir::Value hda : targetOp.getHasDeviceAddrVars()) {
         if (hda.getDefiningOp() == mapOp)
@@ -294,6 +334,26 @@ class MapInfoFinalizationPass
     return false;
   }
 
+  bool isUseDeviceAddr(mlir::omp::MapInfoOp mapOp, mlir::Operation &userOp) {
+    if (auto targetDataOp = llvm::dyn_cast<mlir::omp::TargetDataOp>(userOp)) {
+      for (mlir::Value uda : targetDataOp.getUseDeviceAddrVars()) {
+        if (uda.getDefiningOp() == mapOp)
+          return true;
+      }
+    }
+    return false;
+  }
+
+  bool isUseDevicePtr(mlir::omp::MapInfoOp mapOp, mlir::Operation &userOp) {
+    if (auto targetDataOp = llvm::dyn_cast<mlir::omp::TargetDataOp>(userOp)) {
+      for (mlir::Value udp : targetDataOp.getUseDevicePtrVars()) {
+        if (udp.getDefiningOp() == mapOp)
+          return true;
+      }
+    }
+    return false;
+  }
+
   mlir::omp::MapInfoOp genBoxcharMemberMap(mlir::omp::MapInfoOp op,
                                            fir::FirOpBuilder &builder) {
     if (!op.getMembers().empty())
@@ -358,12 +418,14 @@ class MapInfoFinalizationPass
 
     // TODO: map the addendum segment of the descriptor, similarly to the
     // base address/data pointer member.
-    mlir::Value descriptor = getDescriptorFromBoxMap(op, builder);
+    bool descCanBeDeferred = false;
+    mlir::Value descriptor =
+        getDescriptorFromBoxMap(op, builder, descCanBeDeferred);
 
     mlir::ArrayAttr newMembersAttr;
     mlir::SmallVector<mlir::Value> newMembers;
     llvm::SmallVector<llvm::SmallVector<int64_t>> memberIndices;
-    bool IsHasDeviceAddr = isHasDeviceAddr(op, target);
+    bool IsHasDeviceAddr = isHasDeviceAddr(op, *target);
 
     if (!mapMemberUsers.empty() || !op.getMembers().empty())
       getMemberIndicesAsVectors(
@@ -445,6 +507,10 @@ class MapInfoFinalizationPass
         /*partial_map=*/builder.getBoolAttr(false));
     op.replaceAllUsesWith(newDescParentMapOp.getResult());
     op->erase();
+
+    if (descCanBeDeferred)
+      deferrableDesc.push_back(newDescParentMapOp);
+
     return newDescParentMapOp;
   }
 
@@ -593,6 +659,124 @@ class MapInfoFinalizationPass
     return nullptr;
   }
 
+  void addImplicitDescriptorMapToTargetDataOp(mlir::omp::MapInfoOp op,
+                                              fir::FirOpBuilder &builder,
+                                              mlir::Operation &target) {
+    // Checks if the map is present as an explicit map already on the target
+    // data directive, and not just present on a use_device_addr/ptr, as if
+    // that's the case, we should not need to add an implicit map for the
+    // descriptor.
+    auto explicitMappingPresent = [](mlir::omp::MapInfoOp op,
+                                     mlir::omp::TargetDataOp tarData) {
+      // Verify top-level descriptor mapping is at least equal with same
+      // varPtr, the map type should always be To for a descriptor, which is
+      // all we really care about for this mapping as we aim to make sure the
+      // descriptor is always present on device if we're expecting to access
+      // the underlying data.
+      if (tarData.getMapVars().empty())
+        return false;
+
+      for (mlir::Value mapVar : tarData.getMapVars()) {
+        auto mapOp = llvm::cast<mlir::omp::MapInfoOp>(mapVar.getDefiningOp());
+        if (mapOp.getVarPtr() == op.getVarPtr() &&
+            mapOp.getVarPtrPtr() == op.getVarPtrPtr()) {
+          return true;
+        }
+      }
+
+      return false;
+    };
+
+    // if we're not a top level descriptor with members (e.g. member of a
+    // derived type), we do not want to perform this step.
+    if (!llvm::isa<mlir::omp::TargetDataOp>(target) || op.getMembers().empty())
+      return;
+
+    if (!isUseDeviceAddr(op, target) && !isUseDevicePtr(op, target))
+      return;
+
+    auto targetDataOp = llvm::cast<mlir::omp::TargetDataOp>(target);
+    if (explicitMappingPresent(op, targetDataOp))
+      return;
+
+    mlir::omp::MapInfoOp newDescParentMapOp =
+        builder.create<mlir::omp::MapInfoOp>(
+            op->getLoc(), op.getResult().getType(), op.getVarPtr(),
+            op.getVarTypeAttr(),
+            builder.getIntegerAttr(
+                builder.getIntegerType(64, false),
+                llvm::to_underlying(
+                    llvm::omp::OpenMPOffloadMappingFlags::OMP_MAP_TO |
+                    llvm::omp::OpenMPOffloadMappingFlags::OMP_MAP_ALWAYS)),
+            op.getMapCaptureTypeAttr(), /*varPtrPtr=*/mlir::Value{},
+            mlir::SmallVector<mlir::Value>{}, mlir::ArrayAttr{},
+            /*bounds=*/mlir::SmallVector<mlir::Value>{},
+            /*mapperId*/ mlir::FlatSymbolRefAttr(), op.getNameAttr(),
+            /*partial_map=*/builder.getBoolAttr(false));
+
+    targetDataOp.getMapVarsMutable().append({newDescParentMapOp});
+  }
+
+  void removeTopLevelDescriptor(mlir::omp::MapInfoOp op,
+                                fir::FirOpBuilder &builder,
+                                mlir::Operation *target) {
+    if (llvm::isa<mlir::omp::TargetOp, mlir::omp::TargetDataOp,
+                  mlir::omp::DeclareMapperInfoOp>(target))
+      return;
+
+    // if we're not a top level descriptor with members (e.g. member of a
+    // derived type), we do not want to perform this step.
+    if (op.getMembers().empty())
+      return;
+
+    mlir::SmallVector<mlir::Value> members = op.getMembers();
+    mlir::omp::MapInfoOp baseAddr =
+        mlir::dyn_cast_or_null<mlir::omp::MapInfoOp>(
+            members.front().getDefiningOp());
+    assert(baseAddr && "Expected member to be MapInfoOp");
+    members.erase(members.begin());
+
+    llvm::SmallVector<llvm::SmallVector<int64_t>> memberIndices;
+    getMemberIndicesAsVectors(op, memberIndices);
+
+    // Can skip the extra processing if there's only 1 member as it'd
+    // be the base addresses, which we're promoting to the parent.
+    mlir::ArrayAttr membersAttr;
+    if (memberIndices.size() > 1) {
+      memberIndices.erase(memberIndices.begin());
+      membersAttr = builder.create2DI64ArrayAttr(memberIndices);
+    }
+
+    // VarPtrPtr is tied to detecting if something is a pointer in the later
+    // lowering currently, this at the moment comes tied with
+    // OMP_MAP_PTR_AND_OBJ being applied which breaks the problem this tries to
+    // solve by emitting a 8-byte mapping tied to the descriptor address (even
+    // if we only emit a single map). So we circumvent this by removing the
+    // varPtrPtr mapping, however, a side affect of this is we lose the
+    // additional load from the backend tied to this which is required for
+    // correctness and getting the correct address of the data to perform our
+    // mapping. So we do our load at this stage.
+    // TODO/FIXME: Tidy up the OMP_MAP_PTR_AND_OBJ and varPtrPtr being tied to
+    // if something is a pointer to try and tidy up the implementation a bit.
+    // This is an unfortunate complexity from push-back from upstream. We
+    // could also emit a load at this level for all base addresses as well,
+    // which in turn will simplify the later lowering a bit as well. But first
+    // need to see how well this alteration works.
+    auto loadBaseAddr =
+        builder.loadIfRef(op->getLoc(), baseAddr.getVarPtrPtr());
+    mlir::omp::MapInfoOp newBaseAddrMapOp =
+        builder.create<mlir::omp::MapInfoOp>(
+            op->getLoc(), loadBaseAddr.getType(), loadBaseAddr,
+            baseAddr.getVarTypeAttr(), baseAddr.getMapTypeAttr(),
+            baseAddr.getMapCaptureTypeAttr(), mlir::Value{}, members,
+            membersAttr, baseAddr.getBounds(),
+            /*mapperId*/ mlir::FlatSymbolRefAttr(), op.getNameAttr(),
+            /*partial_map=*/builder.getBoolAttr(false));
+    op.replaceAllUsesWith(newBaseAddrMapOp.getResult());
+    op->erase();
+    baseAddr.erase();
+  }
+
   // This pass executes on omp::MapInfoOp's containing descriptor based types
   // (allocatables, pointers, assumed shape etc.) and expanding them into
   // multiple omp::MapInfoOp's for each pointer member contained within the
@@ -622,6 +806,7 @@ class MapInfoFinalizationPass
       // clear all local allocations we made for any boxes in any prior
       // iterations from previous function scopes.
       localBoxAllocas.clear();
+      deferrableDesc.clear();
 
       // First, walk `omp.map.info` ops to see if any of them have varPtrs
       // with an underlying type of fir.char<k, ?>, i.e a character
@@ -864,6 +1049,36 @@ class MapInfoFinalizationPass
         }
       });
 
+      // Now that we've expanded all of our boxes into a descriptor and base
+      // address map where necessary, we check if the map owner is an
+      // enter/exit/target data directive, and if they are we drop the initial
+      // descriptor (top-level parent) and replace it with the
+      // base_address/data.
+      //
+      // This circumvents issues with stack allocated descriptors bound to
+      // device colliding which in Flang is rather trivial for a user to do by
+      // accident due to the rather pervasive local intermediate descriptor
+      // generation that occurs whenever you pass boxes around different scopes.
+      // In OpenMP 6+ mapping these would be a user error as the tools required
+      // to circumvent these issues are provided by the spec (ref_ptr/ptee map
+      // types), but in prior specifications these tools are not available and
+      // it becomes an implementation issue for us to solve.
+      //
+      // We do this by dropping the top-level descriptor which will be the stack
+      // descriptor when we perform enter/exit maps, as we don't want these to
+      // be bound until necessary which is when we utilise the descriptor type
+      // within a target region. At which point we map the relevant descriptor
+      // data and the runtime should correctly associate the data with the
+      // descriptor and bind together and allow clean mapping and execution.
+      for (auto *op : deferrableDesc) {
+        auto mapOp = llvm::dyn_cast<mlir::omp::MapInfoOp>(op);
+        mlir::Operation *targetUser = getFirstTargetUser(mapOp);
+        assert(targetUser && "expected user of map operation was not found");
+        builder.setInsertionPoint(mapOp);
+        removeTopLevelDescriptor(mapOp, builder, targetUser);
+        addImplicitDescriptorMapToTargetDataOp(mapOp, builder, *targetUser);
+      }
+
       // Wait until after we have generated all of our maps to add them onto
       // the target's block arguments, simplifying the process as there would be
       // no need to avoid accidental duplicate additions.