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Re: [match-and-simplify] express conversions in transform ?

On Tue, Aug 19, 2014 at 12:18 PM, Prathamesh Kulkarni
<> wrote:
> I was wondering how to write transform that involves conversions
> (without using c_expr) ?
> for eg:
> (T1)(~(T2) X)  -> ~(T1) X
> // T1, T2 has same precision and X is an integral type not narrower than T1, T2
> this could be written as:
> (simplify
>   (convert (bit_not (convert@0 integral_op_p@1)))
>   if-expr
>   transform)
> How would the transform be written ?
> with c_expr we could probably write as: (is that correct?)
> (bit_not { fold_convert (type, @1); })

No, unfortunately while that's correct for GENERIC it doesn't work
for GIMPLE code-gen.  c_exprs are required to evaluate to
"atomics", that is, non-expressions.

> I was wondering whether we should have an explicit support for
> conversions (something equivalent of "casting") ?
> sth like:
> (bit_not (cast type @0))

Indeed simply writing

   (bit_not (convert @1))

will make the code-generator "convert" @0 to its own type
(I knew the hack to simply use the first operands type is not
going to work in all cases... ;).  Other conversion operators
include FIX_TRUNC_EXPR (float -> int), FLOAT_EXPR
(int -> float).

For most cases it works by accident as the outermost expression
knows its type via 'type'.

In the cases where we need to specify a type I'd like to avoid
making the type-converting operators take an additional operand.
Instead can we make it use a operator flag-with-argument?  Like

  (bit_not (convert:type @1))

or for the type of a capture (convert:t@2 @1).

We can also put in some more intelligence in automatically
determining the type to convert to.  In your example we
know the bit_not is of type 'type' and this has to match
the type of its operands thus the 'convert' needs to convert
to 'type' as well.  I'm sure there are cases we can't
auto-compute though, like chains of conversions (but not sure
they matter in practice).

So let's try that - Micha will like more intelligence in the generator ;)

I suggest to add a predicate to the generator conversion_op_p
to catch the various converting operators and then assert that
during code-gen we are going to create it with 'type' (outermost
type), not TREE_TYPE (ops[0]).  Then start from the cases
we want to support and special-case (which requires passing
down the type of the enclosing expression).


> in case we want to cast to a type of another capture
> we could use:
> (foo (cast (type @0) @1)) ?  // cast @1 to @0's type
> here type would be an operator that extracts type of the capture.
> Another use of having type as an operator I guess would be for
> type-matching (not sure if it's useful).
> for instance:
> (foo (bar @0 @1) @(type @0)@1)
> that checks if @0, @1 have same types.
> I was wondering on the same lines if we could introduce new
> keyword "precision" analogous to type ?
> (precision @0) would be short-hand for
> So far I found couple patterns in fold_unary_loc with conversions
> in transform:
> (T1)(X p+ Y) into ((T1)X p+ Y)
> (T1)(~(T2) X) -> ~(T1) X
> (T1) (X * Y) -> (T1) X * (T1) Y
> Thanks,
> Prathamesh

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