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

[Kenneth Zadeck <zadeck at naturalbridge dot com>]

Richard Sandiford wrote:
> Kenneth Zadeck <zadeck@naturalbridge.com> writes:
>   
>> Does every one agree that what i am adding is correct?
>>
>> kenny
>> Index: rtl.texi
>> ===================================================================
>> --- rtl.texi	(revision 133159)
>> +++ rtl.texi	(working copy)
>> @@ -1730,15 +1730,21 @@ are in @var{m}.
>>  Sometimes @var{m} is wider than the mode of @var{reg}.  These
>>  @code{subreg} expressions are often called @dfn{paradoxical}.  They are
>>  used in cases where we want to refer to an object in a wider mode but do
>> -not care what value the additional bits have.  The reload pass ensures
>> -that paradoxical references are only made to hard registers.
>> -
>> +not care what value the additional bits have.  The smaller register
>> +always overlaps the least significant bits of the larger register and
>> +the @var{bytenum} is always zero for paradoxical registers (even on big
>> +endian machines).  The reload pass ensures that paradoxical references
>> +are only made to hard registers.
>>     
>
> FWIW, I agree with the first sentence.  I'm not quite sure what you mean
> by the second though.  My understanding is that reload should never
> replace an operand with a subreg of a hard register; it should always
> reduce it to a "reg" rtx.  I think subregs should only appear after
> reload if they are part of an .md pattern (as in spe.md, for example).
>
>   
Note that that sentence was already there.   i added the single sentence
in the middle. 
if you want to suggest new wording for that, i will be happy to add it
to my patch.
>>  The other use of @code{subreg} is to extract the individual registers of
>>  a multi-register value.  Machine modes such as @code{DImode} and
>>  @code{TImode} can indicate values longer than a word, values which
>>  usually require two or more consecutive registers.  To access one of the
>>  registers, use a @code{subreg} with mode @code{SImode} and a
>> -@var{bytenum} offset that says which register.
>> +@var{bytenum} offset that says which register.  In this case, the
>> +@var(bytenum) must align the outer value to a word boundary if the inner
>>     
>        ^^^^^^^^^
> Nit: {bytenum}
>
>   
i will have my best people work on it.
>> +register is a psuedo or to a register boundary if the inner register is
>> +a hard register.  
>>     
>
> As I understand it, this is only true if the _outer_ register is
> word-sized or bigger.  You can have (subreg:QI (reg:DI ...) 3) on
> a 32-bit big-endian target, for example.
>
> (I wish we didn't have subregs of hard regs, but that's a rant for
> another day.  Even if we didn't have them, the rule would still
> apply when "simpilfying" a subreg whose inner register is being
> replaced by a hard register.)
>
>   
I thought that this was implied since the rest of the paragraph is about
the multiword registers.

> Richard
>