  Internal chemical abundance spreads are one of fundamental properties of
globular clusters (GCs) in the Galaxy. In order to understand the origin of
such abundance spreads, we numerically investigate GC formation from massive
molecular clouds (MCs) with fractal structures using our new hydrodynamical
simulations with star formation and feedback effects of supernovae (SNe) and
asymptotic giant branch (AGB) stars. We particularly investigate star formation
from gas chemically contaminated by SNe and AGB stars within MCs with different
initial conditions and environments. The principal results are as follows. GCs
with multiple generation of stars can be formed from merging of hierarchical
star cluster complexes that are developed from high-density regions of fractal
MCs. Feedback effects of SNe and AGB stars can control the formation
efficiencies of stars formed from original gas of MCs and from gas ejected from
AGB stars. The simulated GCs have radial gradients of helium abundances within
the central 3 pc. The original MC masses need to be as large as 10^7 Msun for a
canonical initial stellar mass function (IMF) so that the final masses of stars
formed from AGB ejecta can be 10^5 Msun. Since star formation from AGB ejecta
is rather prolonged (10^8 yr), their formation can be strongly suppressed by
SNe of the stars themselves. This result implies that the so-called mass budget
problem is much more severe than ever thought in the self-enrichment scenario
of GC formation. and thus that IMF for the second generation of stars should be
`top-light'.
