Re: NOP_EXPR vs. CONVERT_EXPR
On Fri, Dec 8, 2023 at 1:24 AM Jeff Law 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
> > 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
Re: NOP_EXPR vs. CONVERT_EXPR
On 12/5/23 07:53, Richard Biener via Gcc wrote:
On Tue, Dec 5, 2023 at 3:54 PM Alexander Monakov via Gcc
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
On Tue, Dec 5, 2023 at 3:54 PM Alexander Monakov via Gcc
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
NOP_EXPR vs. CONVERT_EXPR
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
