piclist@xargs.com wrote:
> A C compiler using a compiled stack looks at the size of the local
> variables a function uses, allocates the appropriate amount of RAM
> from a global buffer, and uses this RAM for the local variables.

That means the local variables are unlikely to be in unbanked memory where
accesses to them are really simple.

> This
> doesn't have the code space overhead of pushing and popping described
> above.

But it does have the additional code space overhead of bank switching before
local variable accesses.

> Nor does it have the (I presume) debugging nightmare of
> declaring the wrong number of local variable bytes to the assembler,
> or forgetting to push and/or pop.

That sort of bug happens very rarely.

> The C compiler will also analyze the function call tree so that
> functions which call other functions can share space in the local
> variable pool without corruption.  For example, if func1 calls func2,
> and each uses two bytes of local variables, the compiler will assign
> bytes 0 and 1 to func1 and bytes 2 and 3 to func2.  But if func1 and
> func2 are mutually exclusive, both will use bytes 0 and 1.  Thus the
> compiler will only allocate the necessary number of bytes for local
> variables, not requiring the (I presume) fixed-size buffer required in
> the approach described above.

But the compiler is forced to make pessimistic assumptions about the call
depth and what local variables can be overlapped.  A stack does this
automatically at run time.  The only issue becomes how much stack space to
allocate.

Each method has its pros and cons.  I don't think you can say up front that
one is inherently better for an arbitrary application.


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