paflof: Silence gcc's -Warray-bounds warning for stack pointers

The SLOF stack pointers - dp/rp - point to the top used element which
means for an empty stack they point to an element 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.

However, with -Warray-bounds enabled, this produces warnings like below:

At the moment SLOF is  gcc produces a warning:
/home/aik/p/slof/slof/paflof.c: In function ‘engine’:
/home/aik/p/slof/slof/paflof.c:84:23: warning: array subscript is below array bounds [-Warray-bounds]
   dp = the_data_stack - 1;
        ~~~~~~~~~~~~~~~^~~

This silences gcc by doing c-cast.

Suggested-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
---

uintptr_t is not used anywhere in SLOF, hence type_u.
---
 slof/paflof.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

On 17.07.2017 05:11, Alexey Kardashevskiy wrote:
> The SLOF stack pointers - dp/rp - point to the top used element which
> means for an empty stack they point to an element 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.
> 
> However, with -Warray-bounds enabled, this produces warnings like below:
> 
> At the moment SLOF is  gcc produces a warning:
> /home/aik/p/slof/slof/paflof.c: In function ‘engine’:
> /home/aik/p/slof/slof/paflof.c:84:23: warning: array subscript is below array bounds [-Warray-bounds]
>    dp = the_data_stack - 1;
>         ~~~~~~~~~~~~~~~^~~
> 
> This silences gcc by doing c-cast.
> 
> Suggested-by: Segher Boessenkool <segher@kernel.crashing.org>
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
> 
> uintptr_t is not used anywhere in SLOF, hence type_u.
> ---
>  slof/paflof.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/slof/paflof.c b/slof/paflof.c
> index 50b4adf..e70f601 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 = (cell *)((type_u)the_data_stack - CELLSIZE);
> +		rp = (cell *)((type_u)handler_stack - CELLSIZE);
>  
>  		// return-address for "evaluate" personality
>  		dummy.a = &&over;

As mentioned in the other e-mail thread already, that solution looks
fine to me.

Reviewed-by: Thomas Huth <thuth@redhat.com>