NOP_EXPR vs. CONVERT_EXPR

NOP_EXPR vs. CONVERT_EXPR

[Alexander Monakov]

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?

Thanks.
Alexander

Re: NOP_EXPR vs. CONVERT_EXPR

[Richard Biener]

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

Re: NOP_EXPR vs. CONVERT_EXPR

[Jeff Law]

On 12/5/23 07:53, Richard Biener via Gcc wrote:
> 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.
I thought someone looked at this a while ago (measured in years) and 
concluded it wasn't actually feasible.  Perhaps because the middle end 
still hands things off to routines that are also used by the FE.

I could see dropping/converting during gimplification with a checker 
that verifies they don't sneak back in.  Then we can start to expunge 
them from gimple passes.  Feels like a gcc-15+ problem to me.

jeff

Re: NOP_EXPR vs. CONVERT_EXPR

[Richard Biener]

On Fri, Dec 8, 2023 at 1:24 AM Jeff Law <jeffreyalaw@gmail.com> wrote:
>
>
>
> On 12/5/23 07:53, Richard Biener via Gcc wrote:
> > 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.
> I thought someone looked at this a while ago (measured in years) and
> concluded it wasn't actually feasible.  Perhaps because the middle end
> still hands things off to routines that are also used by the FE.
>
> I could see dropping/converting during gimplification with a checker
> that verifies they don't sneak back in.  Then we can start to expunge
> them from gimple passes.  Feels like a gcc-15+ problem to me.

It's not so long that I tried this (but really by removing NOP_EXPR) when
I figured the C++ FE at least won't be happy.  The gimplification route
and IL checking so NOP_EXPR doesn't creep back in could work though.

Richard.

> jeff