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[oelgaard <k.b.oelgaard@xxxxxxxxxx>]

I have fixed the issues with variable and exponents != IntValue, so the
above form should compile with FFC now.

I can confirm that it takes 2m46s (166s) to compile the form on my
machine and the bottlenecks are:

expand_indices, time = 114.66s
visit integrand, time =    36.59s

expand_indices is a UFL algorithm, so to make things faster we will have to generate code without calling this.
The time spent in visit integrand might be reduced if we don't call expand_indices on the integrand.

This bug should be fixed and I'm going to close it unless anyone has
objections. We can create an appropriate blueprint to take care of the
compile time issue.

-- 
Problems parsing form file with diff and derivative
https://bugs.launchpad.net/bugs/426512
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Status in FEniCS Form Compiler: Confirmed

Bug description:
FFC has problems parsing the following form file. As I have pointed out in the code, I need to use the following:

from ufl.algorithms import expand_derivatives, strip_variables
L = strip_variables(expand_derivatives(L_temp))
a = strip_variables(expand_derivatives(a_temp))

to get it to compile via FFC. This is not needed for SFC, and while SFC takes a few seconds (without these lines), FFC takes 6m 25s)

Also, FFC doesn't like fractional exponents which is pointed out in bug #423398.

# Finite strain elasticity

# Function space
vector = VectorElement("Lagrange", "tetrahedron", 1)

# Functions
v  = TestFunction(vector)   # Test function
du = TrialFunction(vector)  # Incremental displacement
B  = Function(vector)       # Body force
u  = Function(vector)       # Displacement at previous iteration

# Kinematics
I = Identity(3)
F = I + grad(u).T
C = F.T*F
J = sqrt(det(C))
Fbar = J**(-1./3.)*F  # Does not like fractional exponents
Cbar = J**(-2./3.)*C  # Does not like fractional exponents

E = (J*C - I)/2       # Want to try Cbar here, but using J*C instead
E = variable(E)

# Material law
mu    = Constant("tetrahedron")
lmbda = Constant("tetrahedron")

psi = lmbda/2*(tr(E)**2) + mu*tr(E*E)
S = diff(psi, E)
P = F*S

# Balance of momentum in the reference configuration
L_temp = inner(P, grad(v).T)*dx - inner(B, v)*dx
a_temp = derivative(L_temp, u, du)

# FIXME: Shouldn't need to do the following
from ufl.algorithms import expand_derivatives, strip_variables
L = strip_variables(expand_derivatives(L_temp))
a = strip_variables(expand_derivatives(a_temp))