  What is the origin of quantum computational advantage? Providing answers to
this far-reaching question amounts to identifying the key properties, or
quantum resources, that distinguish quantum computers from their classical
counterparts, with direct applications to the development of quantum devices.
The advent of universal quantum computers, however, relies on error-correcting
codes to protect fragile logical quantum information by robustly encoding it
into symmetric states of a quantum physical system. Such encodings make the
task of resource identification more difficult, as what constitutes a resource
from the logical and physical points of view can differ significantly. Here we
introduce a general framework which allows us to correctly identify quantum
resources in encoded computations, based on phase-space techniques. For a given
quantum code, our construction provides a Wigner function that accounts for how
the symmetries of the code space are contained within the transformations of
the physical space, resulting in an object capable of describing the logical
content of any physical state, both within and outside the code space. We
illustrate our general construction with the Gottesman--Kitaev--Preskill
encoding of qudits with odd dimension. The resulting Wigner function, which we
call the Zak-Gross Wigner function, is shown to correctly identify quantum
resources through its phase-space negativity. For instance, it is positive for
encoded stabilizer states and negative for the bosonic vacuum. We further prove
several properties, including that its negativity provides a measure of magic
for the logical content of a state, and that its marginals are modular
measurement distributions associated to conjugate Zak patches.
