[SLOF] [PATCH] libc: The arguments of puts() can be marked as "const"

[Segher Boessenkool]

Hi!

On Thu, Jul 13, 2017 at 12:42:32PM +1000, Alexey Kardashevskiy wrote:
> On 12/07/17 22:41, Segher Boessenkool wrote:
> > On Wed, Jul 12, 2017 at 06:53:38AM +0200, Thomas Huth wrote:
> >> That's kind of ugly. Why not simply:
> >>
> >> diff --git a/slof/paflof.c b/slof/paflof.c
> >> index 50b4adf..ea3c145 100644
> >> --- a/slof/paflof.c
> >> +++ b/slof/paflof.c
> >> @@ -81,8 +81,8 @@ long engine(int mode, long param_1, long param_2)
> >>                 LAST_ELEMENT(xt_FORTH_X2d_WORDLIST).a = xt_LASTWORD;
> >>
> >>                 // stack-pointers
> >> -               dp = the_data_stack - 1;
> >> -               rp = handler_stack - 1;
> >> +               dp = the_data_stack;
> >> +               rp = handler_stack;
> >>
> >>                 // return-address for "evaluate" personality
> >>                 dummy.a = &&over;
> > 
> > Paflof originally had options to cache 0, 1, or 2 top-of-stack elements.
> > Now it is always 1; it is usually fastest (2 is theoretically a tiny
> > bit better, but a bit worse in practice).
> 
> Sorry I am missing the connection between this 1-cell-cache and the size of
> the stack :)

I left out a bit, and I didn't check either (it actually caches 0 elements
currently).  The # elts cached together with where the stack pointer
points exactly (see below) matters for the code size and speed of the
primitives.

The stack grows upward, and it's a full stack (i.e. the stack pointer
points to the top used element), which means for an empty stack it
points to one below the stack.

This means that for pushing to the stack we can use a store-with-update
instruction (stdu).  This generates good code for most primitives,
better than the other stack pointer offsets.

It doesn't matter all that much; most of the time you are just waiting
for some i/o to happen.


If you want to change it, you should look at DEPTH, PICK, ROLL, and
the things that change stack and THROW/CATCH and such.  And anything
else that "manually" accesses the stacks...


Segher