Re: NOP_EXPR vs. CONVERT_EXPR

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Dec 5, 2023 at 3:54 PM Alexander Monakov via Gcc
<gcc@gcc.gnu.org> wrote:
>
> Greetings,
>
> the definitions for NOP_EXPR and CONVERT_EXPR in tree.def, having survived
> all the way from 1992, currently say:
>
>     /* Represents a conversion of type of a value.
>        All conversions, including implicit ones, must be
>        represented by CONVERT_EXPR or NOP_EXPR nodes.  */
>     DEFTREECODE (CONVERT_EXPR, "convert_expr", tcc_unary, 1)
>
>     /* Represents a conversion expected to require no code to be generated.  */
>     DEFTREECODE (NOP_EXPR, "nop_expr", tcc_unary, 1)
>
> Unfortunately, they are confusing, as in
>
>     float f(double d)
>     {
>         return d;
>     }
>
> the narrowing conversion is represented with NOP_EXPR, and it is definitely
> not a no-op.
>
> Does some clear distinction remain, and is it possible to clarify the
> definitions?

{NOP,CONVERT}_EXPR are interchangeable in the middle-end but
frontends (IIRC the C++ FE mainly) distinguishes them.  So a uniform
documentation might be difficult - in the end we could eventually
drop NOP_EXPR from the middle-end (during gimplification?) and
only use CONVERT_EXPR.  All uses should use CASE_CONVERT
or CONVERT_EXPR_CODE_P which globs both.

Richard.

>
> Thanks.
> Alexander