WIP: Lazy stencil expressions for kernel fusion by bendudson · Pull Request #3401 · boutproject/BOUT-dev

bendudson · 2026-06-21T21:07:08Z

Operators return BinaryExpr types that perform derivatives 'lazily'. Like the single index operators they capture a CoordinatesAccessor to provide access to metrics etc. The difference is that the user code doesn't need to create field accessors or explicitly write the loop, capture variables etc. All that is done automatically if the compiler is smart/sufficiently aggressively optimizing.

Implemented DDZ_C2 and DDZ_C4 operators for Z derivatives. These appear to inline as expected in the Hasegawa-Wakatani example.

Return BinaryExpr types, perform derivatives in Z using CoordinateAccessor to get dz. Appear to inline in Hasegawa-Wakatani example.

github-actions

clang-tidy made some suggestions

github-actions · 2026-06-21T21:09:59Z


    ddt(n) =
-        -bracket(phi, n, bm) + alpha * (nonzonal_phi - nonzonal_n) - kappa * DDZ(phi);
+        -bracket(phi, n, bm) + alpha * (nonzonal_phi - nonzonal_n) - kappa * DDZ_C2(phi);


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

-bracket(phi, n, bm) + alpha * (nonzonal_phi - nonzonal_n) - kappa * DDZ_C2(phi); ^

github-actions · 2026-06-21T21:09:59Z

+  int nz{0};
+
+  template <typename LView, typename RView>
+  BOUT_HOST_DEVICE BOUT_FORCEINLINE BoutReal operator()(int idx, const LView& lhs,


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

BOUT_HOST_DEVICE BOUT_FORCEINLINE BoutReal operator()(int idx, const LView& lhs, ^

github-actions · 2026-06-21T21:09:59Z

+  int nz{0};
+
+  template <typename LView, typename RView>
+  BOUT_HOST_DEVICE BOUT_FORCEINLINE BoutReal operator()(int idx, const LView& lhs,


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

include/bout/stencil_expr.hxx:1:

- #ifndef BOUT_STENCIL_EXPR_HXX + #include "bout/build_config.hxx" + #ifndef BOUT_STENCIL_EXPR_HXX

github-actions · 2026-06-21T21:09:59Z

+  int nz{0};
+
+  template <typename LView, typename RView>
+  BOUT_HOST_DEVICE BOUT_FORCEINLINE BoutReal operator()(int idx, const LView& lhs,


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

include/bout/stencil_expr.hxx:1:

- #ifndef BOUT_STENCIL_EXPR_HXX + #include "bout/bout_types.hxx" + #ifndef BOUT_STENCIL_EXPR_HXX

github-actions · 2026-06-21T21:09:59Z

+
+  template <typename LView, typename RView>
+  BOUT_HOST_DEVICE BOUT_FORCEINLINE BoutReal operator()(int idx, const LView& lhs,
+                                                        const RView&) const {


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

Suggested change

const RView&) const {

const RView& /*unused*/) const {

github-actions · 2026-06-21T21:10:00Z

+
+  template <typename LView, typename RView>
+  BOUT_HOST_DEVICE BOUT_FORCEINLINE BoutReal operator()(int idx, const LView& lhs,
+                                                        const RView&) const {


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

Suggested change

const RView&) const {

const RView& /*unused*/) const {

github-actions · 2026-06-21T21:10:00Z

+inline bout::stencil::DDZExprC2 DDZ_C2(const Field3D& f) {
+  checkData(f);
+
+  const auto region_id = f.getMesh()->getRegionID("RGN_NOBNDRY");


warning: member access into incomplete type 'Mesh' [clang-diagnostic-error]

const auto region_id = f.getMesh()->getRegionID("RGN_NOBNDRY"); ^

Additional context

include/bout/field_data.hxx:47: forward declaration of 'Mesh'

class Mesh; ^

github-actions · 2026-06-21T21:10:00Z

+      f.getLocation(),
+      f.getDirections(),
+      region_id,
+      f.getMesh()->getRegion("RGN_NOBNDRY")};


warning: member access into incomplete type 'Mesh' [clang-diagnostic-error]

f.getMesh()->getRegion("RGN_NOBNDRY")}; ^

Additional context

include/bout/field_data.hxx:47: forward declaration of 'Mesh'

class Mesh; ^

github-actions · 2026-06-21T21:10:00Z

+inline bout::stencil::DDZExprC4 DDZ_C4(const Field3D& f) {
+  checkData(f);
+
+  const auto region_id = f.getMesh()->getRegionID("RGN_NOBNDRY");


warning: member access into incomplete type 'Mesh' [clang-diagnostic-error]

const auto region_id = f.getMesh()->getRegionID("RGN_NOBNDRY"); ^

Additional context

include/bout/field_data.hxx:47: forward declaration of 'Mesh'

class Mesh; ^

github-actions · 2026-06-21T21:10:00Z

+      f.getLocation(),
+      f.getDirections(),
+      region_id,
+      f.getMesh()->getRegion("RGN_NOBNDRY")};


warning: member access into incomplete type 'Mesh' [clang-diagnostic-error]

f.getMesh()->getRegion("RGN_NOBNDRY")}; ^

Additional context

include/bout/field_data.hxx:47: forward declaration of 'Mesh'

class Mesh; ^

Implements Arakawa bracket in X-Z as a BinaryExpr.

Testing the performance impact of runtime dispatch. The hope is to preserve the ability to switch method at runtime. The conditional that selects the method is the same for all iterations, so hopefully good branch prediction and no warp divergence. DDZ_Dispatch(f, method) selects between DDZ_C2 and DDZ_C4 at runtime.

github-actions

clang-tidy made some suggestions

github-actions · 2026-06-23T05:59:48Z


-    ddt(n) =
-        -bracket(phi, n, bm) + alpha * (nonzonal_phi - nonzonal_n) - kappa * DDZ(phi);
+    ddt(n) = -bracket_arakawa(phi, n) + alpha * (nonzonal_phi - nonzonal_n)


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

ddt(n) = -bracket_arakawa(phi, n) + alpha * (nonzonal_phi - nonzonal_n) ^

github-actions · 2026-06-23T05:59:48Z

-    ddt(n) =
-        -bracket(phi, n, bm) + alpha * (nonzonal_phi - nonzonal_n) - kappa * DDZ(phi);
+    ddt(n) = -bracket_arakawa(phi, n) + alpha * (nonzonal_phi - nonzonal_n)
+             - kappa * DDZ_Dispatch(phi, DIFF_C2);


warning: no header providing "DIFF_C2" is directly included [misc-include-cleaner]

- kappa * DDZ_Dispatch(phi, DIFF_C2); ^

github-actions · 2026-06-23T05:59:48Z

+struct DDZ_Dispatch_Op {
+  CoordinatesAccessor coords;
+  int nz{0};
+  DIFF_METHOD method{DIFF_DEFAULT};


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

DIFF_METHOD method{DIFF_DEFAULT}; ^

github-actions · 2026-06-23T05:59:48Z

+struct DDZ_Dispatch_Op {
+  CoordinatesAccessor coords;
+  int nz{0};
+  DIFF_METHOD method{DIFF_DEFAULT};


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

DIFF_METHOD method{DIFF_DEFAULT}; ^

github-actions · 2026-06-23T05:59:48Z

+
+  template <typename LView, typename RView>
+  BOUT_HOST_DEVICE BOUT_FORCEINLINE BoutReal operator()(int idx, const LView& lhs,
+                                                        const RView&) const {


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

Suggested change

const RView&) const {

const RView& /*unused*/) const {

github-actions · 2026-06-23T05:59:48Z

+  BOUT_HOST_DEVICE BOUT_FORCEINLINE BoutReal operator()(int idx, const LView& lhs,
+                                                        const RView&) const {
+    switch (method) {
+    case DIFF_C2:


warning: no header providing "DIFF_C2" is directly included [misc-include-cleaner]

case DIFF_C2: ^

github-actions · 2026-06-23T05:59:48Z

+    switch (method) {
+    case DIFF_C2:
+      return apply_c2(idx, lhs);
+    case DIFF_C4:


warning: no header providing "DIFF_C4" is directly included [misc-include-cleaner]

case DIFF_C4: ^

github-actions · 2026-06-23T05:59:48Z

+
+  if ((method != DIFF_C2) && (method != DIFF_C4)) {
+    throw BoutException("DDZ_Dispatch only supports DIFF_C2 and DIFF_C4, got {:s}",
+                        toString(method));


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

toString(method)); ^

github-actions · 2026-06-23T05:59:49Z

+                                                         const Field3D& g) {
+  checkData(f);
+  checkData(g);
+  ASSERT1_FIELDS_COMPATIBLE(f, g);


warning: no header providing "ASSERT1_FIELDS_COMPATIBLE" is directly included [misc-include-cleaner]

include/bout/stencil_expr.hxx:1:

- #ifndef BOUT_STENCIL_EXPR_HXX + #include "bout/field2d.hxx" + #ifndef BOUT_STENCIL_EXPR_HXX

Stencil expressions: DDZ_C2 and DDZ_C4 operators

c9955c3

Return BinaryExpr types, perform derivatives in Z using CoordinateAccessor to get dz. Appear to inline in Hasegawa-Wakatani example.

bendudson added the work in progress Not ready for merging label Jun 21, 2026

github-actions Bot reviewed Jun 21, 2026

View reviewed changes

bendudson added 2 commits June 22, 2026 15:02

Stencil expressions: bracket_arakawa

acb30ab

Implements Arakawa bracket in X-Z as a BinaryExpr.

github-actions Bot reviewed Jun 23, 2026

View reviewed changes

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