Ram initialization and small c.

[Eric W. Biederman]

Jeremy Jackson <jerj at coplanar.net> writes:

> On Tue, 2003-02-25 at 13:09, Steve Gehlbach wrote:
> 
> > I think you have to cramp your C coding style anyway, to stay within 
> > registers.  The extra scratch area does not help much, with chips such 
> 
> A compiler *made* to compile this way could help a great deal by
> scheduling instructions like an RPN calculator though, so *you* don't
> have to write in spaghetti code.

So far I like the extern inline aspect in that it makes a good proof
of concept that this code can be written in C, and that it is not so
outrageous to expect a compiler to do it.

Steve for a feel of my worries try compiling that code with gcc-3.3.  If
what I saw earlier today is right it won't work because someone has decided
that aggressive inlining is bad thing...

> > BUT, would C's block-scope local variables allow registers to be re-used
> by different local variables, so instead of
> 
> void func (void) {
> register int eax;
> 
> /* use i for one purpose */
> 
> /* use same i for something else */
> }
> 
> you could do
> 
> void func (void) {
> 	{
> 		register int ramsize;
> 		/* use ramsize */
> 	}
> 	{
> 		register int cpuid;
> 		/* use cpuid */
> 	}
> }
> 
> and have both end up using the same register, kind of like a union,
> but still looking more like C than assembler.


Yes.  That is basic live variable analysis.  In general a value not a
variable gets assigned to a register.

The rule is that if a variable is assigned a new value before the
old value is used again.  The variable can be dropped in the
intervening space, and afterwards a new register can be assigned.

It can go as far as a new register assignment every time you change
the value of a variable.

> > as the SiS630, only has three gp regs, little help.  You can't go around 
> > declaring variables willy nilly, you run out of space (registers) no 
> > matter what compiler.
> 
> But for the cases where (in the chipset or whereever) there are scratch
> registers, global extern static variables that are fixed when linking,
> (or define them in an assembler stub with .org or such) would allow them
> to be used easily from C.

That definitely requires some thinking.  When it requires extra registers
to spill a register it can be hard for a compiler.  I am going to
start with the spill free case...

> > So far it is about 400 lines of C, should I attach it?  It compiles 
> > without using the stack (except for a %ebp push/pop which can be deleted).
> 
> Please do.  It would be a good example of how complex the code can be 
> with the register/inline constraints.

It has already shown up on the list but watch out for the inline part
that says it is a pixmap when it should be text plain.

Eric