Re: Global versus Stack variables

"shrey" <shreyas76@gmail.com> writes:




> I was wondering if ppl can give their opinion on the following
> question. What is the impact of a variable being a global or a local
> variable on optimizations. In other words, is it likelier to produce
> better code if a variable is local than global. The reason I ask this
> is I wonder if register allocation is harder if a variable is global
> instead of local. Are there other optimizations which get affected due
> to this consideration.


As John said, it depends on your language and your compiler. But in
C-like languages on modern CPU's (with modern compilers), local
variables tend to be register allocated (unless their address is
taken) where global variables are memory allocated. This means that
access to gobal variables usually require a memory reference, though a
function can sometimes make a temporary copy in a register and use
this for several accesses. It must write back before returning,
calling another function or accessing a variable that might be aliased
with the global variable.


Local variables sometimes also need to be stored across function
calls, though, but you can often reduce the need for this by expoiting
the caller-saves/callee-saves division of registers.


                Torben

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