  This paper presents studies of the performance of several jet-substructure
techniques, which are used to identify hadronically decaying top quarks with
high transverse momentum contained in large-radius jets. The efficiency of
identifying top quarks is measured using a sample of top-quark pairs and the
rate of wrongly identifying jets from other quarks or gluons as top quarks is
measured using multijet events collected with the ATLAS experiment in 20.3
fb$^{-1}$ of 8 TeV proton-proton collisions at the Large Hadron Collider.
Predictions from Monte Carlo simulations are found to provide an accurate
description of the performance. The techniques are compared in terms of signal
efficiency and background rejection using simulations, covering a larger range
in jet transverse momenta than accessible in the dataset. Additionally, a novel
technique is developed that is optimized to reconstruct top quarks in events
with many jets.
