[trunk] Addition to subreg section of rtl.text.

[Kenneth Zadeck]

Richard Sandiford wrote:
> Richard Sandiford <rsandifo@nildram.co.uk> writes:
>   
>>   - A MODE_PARTIAL_INT mode behaves like the corresponding MODE_INT mode.
>>     
> ...except that (as the docs said) an unknown number of bits read as undefined.
>
> Richard
>   
I would add two things to the section.   One is to recommend in the text 
that subregs of hard regs not be used if possible.   The advantages that 
these provide are marginal and there is a reasonable chance that they 
will be outlawed in the future.   The second is to say explicitly that 
subregs of subregs are not legal.  I know that the df code will ignore 
the "middle" subreg.  

Kenny