  Supermassive stars (SMSs) with masses of $M_\ast \simeq 10^4$--$10^5~{\rm
M_\odot}$ are invoked as possible seeds of high-redshift supermassive black
holes, but it remains under debate whether their protostar indeed acquires
sufficient mass via gas accretion overcoming radiative feedback. We investigate
protostellar growth in dynamically heated atomic-cooling haloes (ACHs) found in
recent cosmological simulations, performing three-dimensional radiation
hydrodynamical (RHD) simulations that consider stellar evolution under variable
mass accretion. We find that one of the ACHs feeds the central protostar at
rates exceeding a critical value, above which the star evolves in a cool
bloating phase and hardly produces ionizing photons. Consequently, the stellar
mass reaches $M_\ast \gtrsim 10^4~{\rm M_\odot}$ unimpeded by radiative
feedback. In the other ACH, where the mass supply rate is lower, the star
spends most of its life as a hot main-sequence star, emitting intense ionizing
radiation. Then, the stellar mass growth is terminated around $500~{\rm
M_\odot}$ by photoevaporation of the circumstellar disk. A series of our RHD
simulations provide a formula of the final stellar mass determined either by
stellar feedback or their lifetime as a function of the mass supply rate from
the parent cloud in the absence of stellar radiation. Combining the results
with the statistical properties of SMS-forming clouds in high-redshift quasar
progenitor haloes, we construct a top-heavy mass distribution of primordial
stars over $M_\ast \simeq 100$--$10^5~{\rm M_\odot}$, approximately following a
power-law spectrum of $\propto M_\ast^{-1.3}$ with a steeper decline at $M_\ast
\gtrsim 2 \times 10^4~{\rm M_\odot}$. Their massive BH remnants would be
further fed via the dense debris disk, powering "milli-quasars" with a
bolometric luminosity of $L_{\rm bol}~\gtrsim~10^{43}~{\rm erg~s^{-1}}$.
