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

[Richard Sandiford <rsandifo at nildram dot co dot uk>]

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).

>  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}

> +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.)

Richard