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Re: Setting precision for a PSImode type

On Mon, Mar 5, 2012 at 10:38 AM, Bernd Schmidt <> wrote:
> On 03/05/2012 05:24 PM, Peter Bigot wrote:
>> And is there any reason (other than it doesn't seem to have been done
>> before) to believe PSImode is the wrong way to support a
>> general-purpose 20-bit integral type in gcc?
> If you're using 4.7.0, it should be possible to use FRACTIONAL_INT_MODE
> and get reasonable results. However, it hasn't been tested much, since
> the final bits of the patch series which would have added 40 bit int
> support to the C frontend didn't make it in. See the discussion following
> ?

Thanks; I've updated to 4.7.0.  In this thread I found:

>On 07/01/11 23:18, Bernd Schmidt wrote:
>>> What is the function of having both PARTIAL_INT_MODE and
>> Not having to change all the targets using PARTIAL_INT_MODE immediately
>> to use the better mechanism.
> Also, come to think of it, preventing the rest of the compiler from
> trying to use such a mode in case the target only supports some very
> specific operations on it. A port could choose to use PImode, defined in
> machmode.def (and get __int40_t support), or it could add its own
> private PDImode to use in specific situations only.

Another major difference seems to be that PARTIAL_INT_MODE places the
type in MODE_PARTIAL_INT, while FRACTIONAL_INT_MODE places it in
MODE_INT, and allows it to participate in things like
GET_MODE_WIDER_MODE regardless of whether the fractional mode is
actually available on the target.

I'd really wanted to be able to make general-use 20-bit integers
available to the programmer, but it's looking ugly.  Does it still
make sense to use FRACTIONAL_INT_MODE on a target where there's
essentially an (incomplete) superset of instructions that operate on
the standard registers with 20-bits instead of their original 16-bits?
 Those instructions are only available on a subset of the target
chips, and are enabled by a target-specific user option.  genmodes
doesn't seem to support that constraint.


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