Template expressions with Field3DParallel by bendudson · Pull Request #3430 · boutproject/BOUT-dev

bendudson · 2026-07-10T05:20:36Z

Enables expressions with Field3DParallel to be merged using templates.

If a BinaryExpr is assigned to a Field3D then no calculations are performed on the parallel slices and the result will not have parallel slices.
If a BinaryExpr is assigned to a Field3DParallel then if isFci() the parallel slices will be calculated. If CHECK >= 2 then the assignment will assert that it has slices.
scalars "broadcast" across slices.
The pow and floor functions are now lazy.
Eager arithmetic operators removed from generated_fieldops. In-place operators and FieldPerp operators are still generated (for now).

Unit tests taken from #3428 are passing.

ToDo:

Check that the BinaryExpr changes don't affect performance on GPUs, especially CUDA builds. yup() and ydown() members and Field3D pointers have been added.
Quantify performance change on CPU (optimized) builds.

This was an experiment in expression templates, that has now been replaced by the BinaryExpr machinery.

Views handle yup/ydown slices, evaluating lazy expressions in parallel slices if assigned to a Field3DParallel.

To make it reusable

Implement some clang-tidy suggestions, use FakeMeshFixtureFCI for tests with FCI parallel transform.

github-actions

clang-tidy made some suggestions

There were too many comments to post at once. Showing the first 25 out of 30. Check the log or trigger a new build to see more.

github-actions · 2026-07-10T05:23:48Z

    BOUT_HOST_DEVICE T operator()(int) const { return v; }
+    BOUT_HOST_DEVICE bool hasParallelSlices() const { return false; }
+    BOUT_HOST_DEVICE int numberParallelSlices() const { return 0; }
+    BOUT_HOST_DEVICE View yup(int = 0) const { return *this; }


warning: all parameters should be named in a function [readability-named-parameter]

Suggested change

BOUT_HOST_DEVICE View yup(int = 0) const { return *this; }

BOUT_HOST_DEVICE View yup(int /*unused*/= 0) const { return *this; }

github-actions · 2026-07-10T05:23:49Z

+    BOUT_HOST_DEVICE bool hasParallelSlices() const { return false; }
+    BOUT_HOST_DEVICE int numberParallelSlices() const { return 0; }
+    BOUT_HOST_DEVICE View yup(int = 0) const { return *this; }
+    BOUT_HOST_DEVICE View ydown(int = 0) const { return *this; }


warning: all parameters should be named in a function [readability-named-parameter]

Suggested change

BOUT_HOST_DEVICE View ydown(int = 0) const { return *this; }

BOUT_HOST_DEVICE View ydown(int /*unused*/= 0) const { return *this; }

github-actions · 2026-07-10T05:23:49Z

      this->div = div;
      return *this;
    }
+    BOUT_HOST_DEVICE BOUT_FORCEINLINE bool hasParallelSlices() const { return false; }


warning: method 'hasParallelSlices' can be made static [readability-convert-member-functions-to-static]

Suggested change

BOUT_HOST_DEVICE BOUT_FORCEINLINE bool hasParallelSlices() const { return false; }

BOUT_HOST_DEVICE static BOUT_FORCEINLINE bool hasParallelSlices() { return false; }

github-actions · 2026-07-10T05:23:49Z

      return *this;
    }
+    BOUT_HOST_DEVICE BOUT_FORCEINLINE bool hasParallelSlices() const { return false; }
+    BOUT_HOST_DEVICE BOUT_FORCEINLINE int numberParallelSlices() const { return 0; }


warning: method 'numberParallelSlices' can be made static [readability-convert-member-functions-to-static]

Suggested change

BOUT_HOST_DEVICE BOUT_FORCEINLINE int numberParallelSlices() const { return 0; }

BOUT_HOST_DEVICE static BOUT_FORCEINLINE int numberParallelSlices() { return 0; }

github-actions · 2026-07-10T05:23:49Z

    }
+    BOUT_HOST_DEVICE BOUT_FORCEINLINE bool hasParallelSlices() const { return false; }
+    BOUT_HOST_DEVICE BOUT_FORCEINLINE int numberParallelSlices() const { return 0; }
+    BOUT_HOST_DEVICE BOUT_FORCEINLINE View yup(int = 0) const { return *this; }


warning: all parameters should be named in a function [readability-named-parameter]

Suggested change

BOUT_HOST_DEVICE BOUT_FORCEINLINE View yup(int = 0) const { return *this; }

BOUT_HOST_DEVICE BOUT_FORCEINLINE View yup(int /*unused*/= 0) const { return *this; }

github-actions · 2026-07-10T05:23:49Z

+// when we start using it.
+//
+// Can turn off the ability to do the transform. Should still be valid
+class MockParallelTransform : public ParallelTransform {


warning: class 'MockParallelTransform' defines a default destructor but does not define a copy constructor, a copy assignment operator, a move constructor or a move assignment operator [cppcoreguidelines-special-member-functions]

class MockParallelTransform : public ParallelTransform { ^

github-actions · 2026-07-10T05:23:50Z

+// when we start using it.
+//
+// Can turn off the ability to do the transform. Should still be valid
+class MockParallelTransform : public ParallelTransform {


warning: no header providing "ParallelTransform" is directly included [misc-include-cleaner]

tests/unit/fake_mesh.hxx:2:

- #include <bout/boundary_region.hxx> + #include "bout/paralleltransform.hxx" + #include <bout/boundary_region.hxx>

github-actions · 2026-07-10T05:23:50Z

+public:
+  MockParallelTransform(Mesh& mesh, bool allow_transform_)
+      : ParallelTransform(mesh), allow_transform(allow_transform_) {}
+  ~MockParallelTransform() = default;


warning: annotate this function with 'override' or (rarely) 'final' [cppcoreguidelines-explicit-virtual-functions]

Suggested change

~MockParallelTransform() = default;

~MockParallelTransform() override = default;

github-actions · 2026-07-10T05:23:50Z

+      : ParallelTransform(mesh), allow_transform(allow_transform_) {}
+  ~MockParallelTransform() = default;
+
+  void calcParallelSlices(Field3D&) override {}


warning: all parameters should be named in a function [readability-named-parameter]

Suggested change

void calcParallelSlices(Field3D&) override {}

void calcParallelSlices(Field3D& /*f*/) override {}

github-actions · 2026-07-10T05:23:50Z

+
+  bool canToFromFieldAligned() const override { return allow_transform; }
+
+  bool requiresTwistShift(bool, YDirectionType) override { return false; }


warning: all parameters should be named in a function [readability-named-parameter]

Suggested change

bool requiresTwistShift(bool, YDirectionType) override { return false; }

bool requiresTwistShift(bool /*twist_shift_enabled*/, YDirectionType /*ytype*/) override { return false; }

More unit tests

github-actions

clang-tidy made some suggestions

github-actions · 2026-07-10T05:59:52Z

 namespace bout::detail {
+template <typename T>
+using UnaryFieldResult_t =
+    std::conditional_t<std::is_same_v<std::decay_t<T>, ::Field3DParallel>, ::Field3D,


warning: no header providing "std::conditional_t" is directly included [misc-include-cleaner]

include/bout/field.hxx:26:

- class Field; + #include <type_traits> + class Field;

github-actions · 2026-07-10T05:59:52Z

 namespace bout::detail {
+template <typename T>
+using UnaryFieldResult_t =
+    std::conditional_t<std::is_same_v<std::decay_t<T>, ::Field3DParallel>, ::Field3D,


warning: no header providing "std::decay_t" is directly included [misc-include-cleaner]

= ^

github-actions · 2026-07-10T05:59:52Z

+
+  void checkInputGrid() override {}
+
+  Field3D fromFieldAligned(const Field3D& f, const std::string&) override {


warning: all parameters should be named in a function [readability-named-parameter]

Suggested change

Field3D fromFieldAligned(const Field3D& f, const std::string&) override {

Field3D fromFieldAligned(const Field3D& f, const std::string& /*region*/) override {

github-actions · 2026-07-10T05:59:52Z

+    return -f;
+  }
+
+  FieldPerp fromFieldAligned(const FieldPerp& f, const std::string&) override {


warning: all parameters should be named in a function [readability-named-parameter]

Suggested change

FieldPerp fromFieldAligned(const FieldPerp& f, const std::string&) override {

FieldPerp fromFieldAligned(const FieldPerp& f, const std::string& /*region*/) override {

github-actions · 2026-07-10T05:59:52Z

+    return -f;
+  }
+
+  Field3D toFieldAligned(const Field3D& f, const std::string&) override {


warning: all parameters should be named in a function [readability-named-parameter]

Suggested change

Field3D toFieldAligned(const Field3D& f, const std::string&) override {

Field3D toFieldAligned(const Field3D& f, const std::string& /*region*/) override {

github-actions · 2026-07-10T05:59:52Z

+    }
+    return -f;
+  }
+  FieldPerp toFieldAligned(const FieldPerp& f, const std::string&) override {


warning: all parameters should be named in a function [readability-named-parameter]

Suggested change

FieldPerp toFieldAligned(const FieldPerp& f, const std::string&) override {

FieldPerp toFieldAligned(const FieldPerp& f, const std::string& /*region*/) override {

github-actions · 2026-07-10T05:59:52Z

+  }
+
+private:
+  const bool allow_transform;


warning: member 'allow_transform' of type 'const bool' is const qualified [cppcoreguidelines-avoid-const-or-ref-data-members]

const bool allow_transform; ^

If the parallel transform is not FCI then parallel slices are not calculated on assignment to Field3DParallel.

Makes the `floor` function lazy, so it can be inlined into template expressions.

Arithmetic operators now use the lazy `BinaryExpr` expressions.

Remove unnecessary data members that dublicate the Field3D::View base.

github-actions

clang-tidy made some suggestions

github-actions · 2026-07-10T23:55:05Z

+                static_cast<int>(numberParallelSlices())};
+  }
+  operator View() const {
+    return View{const_cast<BoutReal*>(&data[0]), yup_fields.data(), ydown_fields.data(),


warning: do not use const_cast to remove const qualifier [cppcoreguidelines-pro-type-const-cast]

return View{const_cast<BoutReal*>(&data[0]), yup_fields.data(), ydown_fields.data(), ^

github-actions · 2026-07-10T23:55:05Z

+    /// Not a DEVICE function because it dereferences a Field3D pointer
+    BOUT_FORCEINLINE View yup(int slice = 0) const {
+      ASSERT2(slice < base.num_parallel_slices);
+      ASSERT2(base.yup_fields[slice].isAllocated());


warning: pointer arithmetic on polymorphic object of type 'Field3D' can result in undefined behavior if the dynamic type differs from the pointer type [bugprone-pointer-arithmetic-on-polymorphic-object]

ASSERT2(base.yup_fields[slice].isAllocated()); ^

Additional context

include/bout/assert.hxx:50: expanded from macro 'ASSERT2'

if (!(condition)) { \ ^

github-actions · 2026-07-10T23:55:05Z

+    BOUT_FORCEINLINE View yup(int slice = 0) const {
+      ASSERT2(slice < base.num_parallel_slices);
+      ASSERT2(base.yup_fields[slice].isAllocated());
+      return View{static_cast<Field3D::View>(base.yup_fields[slice])};


warning: pointer arithmetic on polymorphic object of type 'Field3D' can result in undefined behavior if the dynamic type differs from the pointer type [bugprone-pointer-arithmetic-on-polymorphic-object]

return View{static_cast<Field3D::View>(base.yup_fields[slice])}; ^

github-actions · 2026-07-10T23:55:05Z

+    /// Not a DEVICE function because it dereferences a Field3D pointer
+    BOUT_FORCEINLINE View ydown(int slice = 0) const {
+      ASSERT2(slice < base.num_parallel_slices);
+      ASSERT2(base.ydown_fields[slice].isAllocated());


warning: pointer arithmetic on polymorphic object of type 'Field3D' can result in undefined behavior if the dynamic type differs from the pointer type [bugprone-pointer-arithmetic-on-polymorphic-object]

ASSERT2(base.ydown_fields[slice].isAllocated()); ^

Additional context

include/bout/assert.hxx:50: expanded from macro 'ASSERT2'

if (!(condition)) { \ ^

github-actions · 2026-07-10T23:55:05Z

+    BOUT_FORCEINLINE View ydown(int slice = 0) const {
+      ASSERT2(slice < base.num_parallel_slices);
+      ASSERT2(base.ydown_fields[slice].isAllocated());
+      return View{static_cast<Field3D::View>(base.ydown_fields[slice])};


warning: pointer arithmetic on polymorphic object of type 'Field3D' can result in undefined behavior if the dynamic type differs from the pointer type [bugprone-pointer-arithmetic-on-polymorphic-object]

return View{static_cast<Field3D::View>(base.ydown_fields[slice])}; ^

bendudson and others added 6 commits July 9, 2026 13:48

Delete unneededed expr.hxx

23ddf98

This was an experiment in expression templates, that has now been replaced by the BinaryExpr machinery.

Field3DParallel BinaryExpr templates

49774de

Views handle yup/ydown slices, evaluating lazy expressions in parallel slices if assigned to a Field3DParallel.

Add unit test to ensure Field3DParallel works for products with FCI

470abe4

Allow FCI fixture

c23f2d5

Move MockParallelTransform

356fd0c

To make it reusable

test_field3d: Tidying, define FakeMeshFixtureFCI

8f66c00

Implement some clang-tidy suggestions, use FakeMeshFixtureFCI for tests with FCI parallel transform.

github-actions Bot reviewed Jul 10, 2026

View reviewed changes

Functions of Field3DParallel preserve slices

ceaaf98

More unit tests

github-actions Bot reviewed Jul 10, 2026

View reviewed changes

bendudson mentioned this pull request Jul 10, 2026

Fixes and unit tests for Field3DParallel #3428

Open

bendudson added 3 commits July 10, 2026 08:24

Field3DParallel unit test

b2f52cd

If the parallel transform is not FCI then parallel slices are not calculated on assignment to Field3DParallel.

Lazy pow function on fields

858c5c1

floor: Return BinaryExpr

c808fa7

Makes the `floor` function lazy, so it can be inlined into template expressions.

bendudson requested a review from dschwoerer July 10, 2026 17:58

bendudson added 3 commits July 10, 2026 13:12

generated_fieldops: Remove eager functions

c7493e4

Arithmetic operators now use the lazy `BinaryExpr` expressions.

Field3DParallel::View simplify

d41d93c

Remove unnecessary data members that dublicate the Field3D::View base.

manual: Updates for Field3DParallel expressions

3c6565d

github-actions Bot reviewed Jul 10, 2026

View reviewed changes

	BOUT_HOST_DEVICE View yup(int = 0) const { return *this; }
	BOUT_HOST_DEVICE View yup(int /unused/= 0) const { return *this; }

	BOUT_HOST_DEVICE View ydown(int = 0) const { return *this; }
	BOUT_HOST_DEVICE View ydown(int /unused/= 0) const { return *this; }

	BOUT_HOST_DEVICE BOUT_FORCEINLINE bool hasParallelSlices() const { return false; }
	BOUT_HOST_DEVICE static BOUT_FORCEINLINE bool hasParallelSlices() { return false; }

	BOUT_HOST_DEVICE BOUT_FORCEINLINE int numberParallelSlices() const { return 0; }
	BOUT_HOST_DEVICE static BOUT_FORCEINLINE int numberParallelSlices() { return 0; }

	BOUT_HOST_DEVICE BOUT_FORCEINLINE View yup(int = 0) const { return *this; }
	BOUT_HOST_DEVICE BOUT_FORCEINLINE View yup(int /unused/= 0) const { return *this; }

	~MockParallelTransform() = default;
	~MockParallelTransform() override = default;

	void calcParallelSlices(Field3D&) override {}
	void calcParallelSlices(Field3D& /f/) override {}


		bool canToFromFieldAligned() const override { return allow_transform; }

		bool requiresTwistShift(bool, YDirectionType) override { return false; }

	bool requiresTwistShift(bool, YDirectionType) override { return false; }
	bool requiresTwistShift(bool /twist_shift_enabled/, YDirectionType /ytype/) override { return false; }


		void checkInputGrid() override {}

		Field3D fromFieldAligned(const Field3D& f, const std::string&) override {

	FieldPerp fromFieldAligned(const FieldPerp& f, const std::string&) override {
	FieldPerp fromFieldAligned(const FieldPerp& f, const std::string& /region/) override {

	Field3D toFieldAligned(const Field3D& f, const std::string&) override {
	Field3D toFieldAligned(const Field3D& f, const std::string& /region/) override {

	FieldPerp toFieldAligned(const FieldPerp& f, const std::string&) override {
	FieldPerp toFieldAligned(const FieldPerp& f, const std::string& /region/) override {

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bendudson commented Jul 10, 2026 •

edited

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