Mahmood's PS:
"HLL compilers rely heavily on the stack"

In this context, we should say that HLL (High-Level Language; eg, 'C') compilers normally rely heavily on the stack.
As far as a normal HLL implementation is concerned, the 8051 doesn't have a stack worth speaking of - therefore 'C' implementations for the 8051 have to play all sorts of cunning tricks to cope with this fundamental limitation of the architecture.
The following Thread is a prime example of the kind of problem that arises:
[C51] : RECURSIVE CALL TO SEGMENT
(the discussion continues on the Keil forum)