RFC: Allowing target specific extensions to RTL checking

[Nick Clifton]

Hi Richard,

>>    So is there some way that I can say that for the RX target the
>>    frame_related bit on CONST_INTs can be set ?
>
> No, there isn't.  Please describe what you're trying to achieve here.
> With no further information it sounds like you want
>
>    (const:SI (unspec:SI [(const_int x)] UNSPEC_SOMETHING))

Hmm, maybe that is what I need.

In essence I am trying to fix a bug with the implementation of the 
-max-constant-size feature of the RX port.  This feature can be used to 
force large constants to be dumped into a constant pool and loaded 
indirectly, rather than being encoded in each instruction as needed. 
This can lead to smaller/faster code, but there is no simple heuristic 
to determine when this will work, hence the command line switch.

The bug is that if -mmax-constant-size is being used and a function 
needs a large stack frame the prologue generation code will emit an 
ADDSI3 insn without allowing for the fact that the constant value it is 
using is no longer valid.  What I wanted was a way to be able to mark 
constants used by function prologues and epilogues as being special and 
exempt from the limits checking performed by the LEGITIMATE_CONSTANT_P 
macro.

I found that I could set the RTX_FRAME_RELATED_P bit on the stack frame 
adjust constants and then detect them in rx_is_legitimate_constant(). 
But this breaks when I build with --enable-checking.  Hence my original 
email to find out if there is a better way to do this.

Cheers
   Nick