Re: Generating gimple assign stmt that changes sign

[Marc Glisse <marc dot glisse at inria dot fr>]

On Tue, 22 May 2018, 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))))

Not sure, but we may want a few more restrictions on this transformation.

> 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.

You'll probably want to patch genmatch.c (near get_operand_type maybe?) so 
it doesn't try to guess that the type of absu is the same as its argument. 
You can also specify a type in transformations, look for :utype or :etype 
in match.pd.

--
Marc Glisse