On Wed, Jul 20, 2011 at 8:04 PM, Eric Botcazou <ebotca...@adacore.com> wrote:
>> Another improvement is introduction of SUBREG_PROMOTED_UNSIGNED_P
>> predicate in the same area to handle paradoxical SUBREGs. When SUBREG
>> satisfies this predicate, the compiler guarantees, that excess bits
>> are zero (see the documentation). This is exaclty what we want in
>> registers that form the address. In theory, we can allow registers of
>> any integer mode here, but in practice only DImode subreg of SImode
>> register is important.
>
> Note that SUBREG_PROMOTED_UNSIGNED_P wasn't designed for paradoxical subregs,
> but for regular subregs (typically of word-sized objects). You should check
> that the ones created for x32 (because of POINTERS_EXTEND_UNSIGNED I guess)
> are legitimate.
Hm, the documentation says:
Paradoxical subregs
When M1 is strictly wider than M2, the `subreg' expression is
called "paradoxical". The canonical test for this class of
`subreg' is:
GET_MODE_SIZE (M1) > GET_MODE_SIZE (M2)
Paradoxical `subreg's can be used as both lvalues and rvalues.
When used as an lvalue, the low-order bits of the source value
are stored in REG and the high-order bits are discarded.
When used as an rvalue, the low-order bits of the `subreg' are
taken from REG while the high-order bits may or may not be
defined.
The high-order bits of rvalues are in the following
circumstances:
* `subreg's of `mem' When M2 is smaller than a word, the
macro `LOAD_EXTEND_OP', can control how the high-order
bits are defined.
* `subreg' of `reg's The upper bits are defined when
`SUBREG_PROMOTED_VAR_P' is true.
`SUBREG_PROMOTED_UNSIGNED_P' describes what the upper
bits hold. Such subregs usually represent local
variables, register variables and parameter pseudo
variables that have been promoted to a wider mode.
I have looked at example in rs6000.c, the only target that uses
SUBREG_PROMOTED_UNSIGNED_P. Looking at other sources, S_P_U_P is used
in conjunction with SUBREG_PROMOTED_VAR. It looks to me that using the
combination should be OK to determine if subreg is correct.
Uros.
