[Bug 1607] __RTEMS_SIZEOF_VOID_P__ flawed design

[bugzilla-daemon at rtems.org]

https://www.rtems.org/bugzilla/show_bug.cgi?id=1607

--- Comment #8 from Sebastian Huber <sebastian.huber at embedded-brains.de> 2010-07-08 02:45:16 CDT ---
(In reply to comment #7)
> (In reply to comment #6)
> > 1. We need this per CPU structure to support multi-core systems.
> > 2. We have to access fields of this CPU structure in assembly code.
> Not quite - Your design does not allow otherwise.
> 
> You could redesign your code in such a way it doesn't have these
> limitations/restriction attached, e.g. 
> * by rewriting your code in inline-asm (There you can utilize sizeof())

Inline-asm has be be within a function body?  The exception entry is on many
architectures not a function call.  Usually you have to set up an environment
in which a call to C functions is allowed.  This may require that you have to
switch the stack pointer.  The decision to switch the stack or not usually
depends on a variable within this per CPU structure (the ISR nest level).  Such
stack operations are difficult to get right with inline-asm.  Writing the
exception code is usually hard to get right and inline-asm adds a considerable
complexity of its own.  It is also hard to test.  The inline-asm may depend on
the compiler version.

> or
> * by using helper const variables being set up in C and refer to them in asm or
> inline-asm

This is a performance penalty in a time critical section.  Every instruction
here contributes to the worst case interrupt latency.

> or
> * by re-writing everything in C, such that only very few atomic ops (functions)
> would have to be needed.

This is not possible on many architectures.  The exception handling is usually
not aware of an ABI.

> 
> There probably are many more ways.

Probably maybe.

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