On 4/26/19 3:09 PM, Segher Boessenkool wrote:
> On Fri, Apr 26, 2019 at 06:06:37PM +0100, Richard Earnshaw (lists) wrote:
>> On 26/04/2019 17:08, Jeff Law wrote:
>>>> So is that valid RTL (DI extract of SI value)? Why wouldn't that just
>>>> use a paradoxical subreg to widen the register being extracted?
>>> [ ... ]
>>> And for completeness, the patch also survived regression testing
>>> aarch64be where it fixes the lsl_asr_sbfiz test.
>>>
>>> I think the big question here is whether or not we consider this valid RTL.
>>>
>>
>> The documentation for sign_extract says:
>>
>> @item (sign_extract:@var{m} @var{loc} @var{size} @var{pos})
>>
>> ...
>>
>> If @var{loc} is in memory, its mode must be a single-byte integer mode.
>> If @var{loc} is in a register, the mode to use is specified by the
>> operand of the @code{insv} or @code{extv} pattern
>> (@pxref{Standard Names}) and is usually a full-word integer mode,
>> which is the default if none is specified.
>>
>> So it's a little unclear to me whether the mode of loc is ignored for
>> registers, or if it must match the mode of the extract.
>
> It must use the mode the extv pattern says to use. But you don't *have*
> such a pattern here, you have extv<mode> instead.
Right.
>
> It makes most sense if the mode for extv<mode> is the same both in and out
> (it has only one mode in the pattern name, to start with), and for
> sign_extract to be similar. The docs aren't quite clear, but defining it
> to have multiple modes doesn't really solve anything afaics, subregs work
> just fine here?
You'd have to scatter them in the MD file. That's generally frowned upon.
My argument is that we have very clear semantics here. We're taking a
field from an object in a mode. We zero or sign extract it to the mode
of the destination. Two modes actually make reasonable sense here.
Think of it like zero_extend or sign_extend, but for a bitfield.
jeff
