Re: [Qemu-devel] [PATCH] const / static (against current CVS)

[Paul Brook <paul@codesourcery.com>]

> Probably more important is to make sure none constant data structures
> are done on the stack.  There is no good reason why any code page
> should be read-write.

Huh? this is nonsense.

You have three segements in an application (ignoring dynamic heap allocated 
memory):

The RO segment that contains code and readonly data. This is typically 
implemented as a readonly file mapping shared by multiple applications.

The RW segment contains read/write data, some of which may be initialized by 
data stored in the executable file, the rest is zero-initialized at startup.

The Stack is readwrite, unititialized, and typically allocated dynamically at 
runtime.

The compiler never puts readwrite objects in th RO segment. If it does you've 
got a buggy toolchain or build system.

Making global data readonly is a small win because it means it can be shared 
by multiple instances of the same application.

Moving global RW data onto the stack isn't neccly a win. Most systems have a 
relatively small limit on stack size, so putting large objects on the stack 
is a bad idea.

Contrary to popular belief the "const" qualifier on pointers has absolutely no 
effect on optimization. It's simply a debugging aid so the compiler will 
generate an error if you accidentally assign to it. It's perfectly legal to 
cast a (const char *) to a (char *) then dereference and write to it, 
provided the object the object it points to is modifiable.

Paul