Revise derivative function call; Add default derivative version

src/equations/compressible_euler_2d.jl

@@ -1405,7 +1405,8 @@ end
 end
 
 # Transformation from conservative variables u to d(p)/d(u)
-@inline function pressure(u, equations::CompressibleEulerEquations2D, derivative::True)
+@inline function variable_derivative(::typeof(pressure),
+                                     u, equations::CompressibleEulerEquations2D)
     rho, rho_v1, rho_v2, rho_e = u
 
     v1 = rho_v1 / rho

src/equations/equations.jl

@@ -369,6 +369,12 @@ of the correct length `nvariables(equations)`.
 """
 function energy_internal end
 
+# Default implementation of derivation for `variable`. Used for subcell limiting.
+# Implementing a derivative function for a specific function improves the performance.
+@inline function variable_derivative(variable, u, equations)
+    return ForwardDiff.gradient(x -> variable(x, equations), u)
+end
+
 ####################################################################################################
 # Include files with actual implementations for different systems of equations.
 

src/equations/ideal_glm_mhd_2d.jl

@@ -1063,7 +1063,8 @@ end
 end
 
 # Transformation from conservative variables u to d(p)/d(u)
-@inline function pressure(u, equations::IdealGlmMhdEquations2D, derivative::True)
+@inline function variable_derivative(::typeof(pressure),
+                                     u, equations::IdealGlmMhdEquations2D)
     rho, rho_v1, rho_v2, rho_v3, rho_e, B1, B2, B3, psi = u
 
     v1 = rho_v1 / rho

src/solvers/dgsem_tree/subcell_limiters_2d.jl

@@ -466,7 +466,7 @@ end
 # Goal and d(Goal)d(u) function
 @inline goal_function(variable, bound, u, equations) = bound - variable(u, equations)
 @inline function dgoal_function(variable, u, dt, antidiffusive_flux, equations)
-    -dot(variable(u, equations, True()), dt * antidiffusive_flux)
+    -dot(variable_derivative(variable, u, equations), dt * antidiffusive_flux)
 end
 
 # Final checks