[PATCH 02/10] compiler.h: add is_const() as a replacement of __is_constexpr()

[Vincent Mailhol]

-CC: Martin Uecker <Martin.Uecker at med.uni-goettingen.de>
+CC: Martin Uecker <muecker at gwdg.de>
(seems that Martin changed his address)

On Thu. 5 Dec. 2024 at 03:39, David Laight <David.Laight at aculab.com> wrote:
> > Sent: 02 December 2024 17:33
> >
> > From: Vincent Mailhol <mailhol.vincent at wanadoo.fr>
> >
> > __is_constexpr(), while being one of the most glorious one liner hack
> > ever witnessed by mankind, is overly complex. Following the adoption
> > of C11 in the kernel, this macro can be simplified through the use of
> > a _Generic() selection.
>
> You should give credit to some of the earlier patches that do the same.
> I'm sure there were some related ones from Linus - not applied yet.

ACK. Would adding a suggested--by Linus tag solve your concern?

> > First, split the macro in two:
> >
> >   - __is_const_zero(x): an helper macro; tells whether x is the
> >     integer constant expression 0 or something else.
> >
> >   - is_const(x): replacement of __is_constexpr(); tells whether x is a
> >     integer constant expression.
> >
> > The split serves two purposes: first make it easier to understand;
> > second, __is_const_zero() will be reused as a building block for other
> > is_const_*() macros that will be introduced later on.
> >
> > The core principle of __is_constexpr() to abuse the return type of the
> > ternary operator remains, but all the surrounding sizeof() hack
> > disappear.
> >
> > On a side note, while not relevant to the kernel, __is_constexpr()
> > relied on the GNU extension that sizeof(void) is 1. const_expr() does
> > not use any GNU extensions, making it ISO C compliant.
> >
> > __is_constexpr() is temporarily kept and will be removed once all its
> > users get migrated to is_const() (or one of its friend).
> >
> > Signed-off-by: Vincent Mailhol <mailhol.vincent at wanadoo.fr>
> > ---
> >  include/linux/compiler.h | 41 +++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 41 insertions(+)
> >
> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index a2a56a50dd85227a4fdc62236a2710ca37c5ba52..30ce06df4153cfdc0fad9bc7bffab9097f8b0450 100644
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -316,6 +316,47 @@ static inline void *offset_to_ptr(const int *off)
> >  #define statically_true(x) (__builtin_constant_p(x) && (x))
> >  #define statically_false(x) (__builtin_constant_p(x) && (x) == 0)
> >
> > +/*
> > + * Whether x is the integer constant expression 0 or something else.
> > + *
> > + * Details:
> > + *   - The C11 standard defines in §6.3.2.3.3
> > + *       (void *)<integer constant expression with the value 0>
> > + *     as a null pointer constant (c.f. the NULL macro).
> > + *   - If x evaluates to the integer constant expression 0,
> > + *       (void *)(x)
> > + *     is a null pointer constant. Else, it is a void * expression.
> > + *   - In a ternary expression:
> > + *       condition ? operand1 : operand2
> > + *     if one of the two operands is of type void * and the other one
> > + *     some other pointer type, the C11 standard defines in §6.5.15.6
> > + *     the resulting type as below:
> > + *       if one operand is a null pointer constant, the result has the
> > + *       type of the other operand; otherwise [...] the result type is
> > + *       a pointer to an appropriately qualified version of void.
> > + *   - As such, in
> > + *       0 ? (void *)(x) : (char *)0
> > + *     if x is the integer constant expression 0, operand1 is a null
> > + *     pointer constant and the resulting type is that of operand2:
> > + *     char *. If x is anything else, the type is void *.
> > + *   - The (long) cast silences a compiler warning for when x is not 0.
> > + *   - Finally, the _Generic() dispatches the resulting type into a
> > + *     Boolean.
>
> The comment is absolutely excessive.
> I'm sure I managed about 2 lines in one of the patches I did.

I think that Linus made it  clear in:

  https://lore.kernel.org/all/CAHk-=wgfpLdt7SFFGcByTfHdkvv7AEa3MDu_s_W1kfOxQs49pw@mail.gmail.com/

that this deserves a detailed comment.

The details block in the current __is_constexpr() is 37 lines long,
the details block in __is_const_zero() takes 22 lines. So I would
argue that I made things better.

Unless more people share your concern, I am planning to keep this comment as-is.

> > + *
> > + * Glory to Martin Uecker <Martin.Uecker at med.uni-goettingen.de>
>
> IIRC Martin has agreed in the past that the accreditation can
> be removed - especially since it refers to the 'sizeof (void)' trick.

I tried to look for such message:

  https://lore.kernel.org/all/?q=f%3A%22martin+uecker%22+__is_constexpr

but couldn't find it. Do you have the link?

@Martin, do you agree that I remove the accreditation?

> > + */
> > +#define __is_const_zero(x) \
> > +     _Generic(0 ? (void *)(long)(x) : (char *)0, char *: 1, void *: 0)
> > +
> > +/*
> > + * Returns a constant expression while determining if its argument is a
> > + * constant expression, most importantly without evaluating the argument.
>
> You need to differentiate between a 'constant integer expression'
> and a 'compile time constant'.

OK. This one was just copied from the previous __is_constexpr(). I will apply
"s/constant expression/constant integer expression/g" in v2.

> > + *
> > + * If getting a constant expression is not relevant to you, use the more
> > + * powerful __builtin_constant_p() instead.
>
> __builtin_constant_p() is not 'more powerful' it is testing for
> something different.

I meant to say that __builtin_constant_p() is more powerful at
constant folding. But I agree that the comment is not clear.

What about this?

  If getting a constant integer expression is not relevant to you, use
  __builtin_constant_p() which not only returns true if the argument
  is an integer constant expression, but also if it is a compile time
  constant.


Yours sincerely,
Vincent Mailhol