Re: Generating gimple assign stmt that changes sign

[Kugan Vivekanandarajah <kugan dot vivekanandarajah at linaro dot org>]

Hi Jeff,

Thanks for the prompt reply.

On 22 May 2018 at 09:10, Jeff Law <law@redhat.com> wrote:
> On 05/21/2018 04:50 PM, Kugan Vivekanandarajah wrote:
>> Hi,
>>
>> I am looking to introduce ABSU_EXPR and that would create:
>>
>> unsigned short res = ABSU_EXPR (short);
>>
>> Note that the argument is signed and result is unsigned. As per the
>> review, I have a match.pd entry to generate this as:
>> (simplify (abs (convert @0))
>>  (if (ANY_INTEGRAL_TYPE_P (TREE_TYPE (@0)))
>>   (convert (absu @0))))
>>
>>
>> Now when gimplifying the converted tree, how do we tell that ABSU_EXPR
>> will take a signed arg and return unsigned. I will have other match.pd
>> entries so this will be generated while in gimple.passes too. Should I
>> add new functions in gimple.[h|c] for this.
>>
>> Is there any examples I can refer to. Conversion expressions seems to
>> be the only place where sign can change in gimple assignment but they
>> are very specific.
> What's the value in representing ABSU vs a standard ABS followed by a
> conversion?

It is based on PR https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64946.
Specifically, comment 13.

>
> You'll certainly want to do verification of the type signedness in the
> gimple verifier.
I am doing it and it is failing now.

>
> In general the source and destination types have to be the same.
> Conversions are the obvious exception.  There's a few other nodes that
> have more complex type rules (MEM_REF, COND_EXPR and a few others).  But
> I don't think they're necessarily going to be helpful.


Thanks,
Kugan
>
> jeff