[PATCH 03/10] compiler.h: add is_const_true() and is_const_false()

[Vincent Mailhol]

On Thu. 5 Dec 2024 at 03:48, David Laight <David.Laight at aculab.com> wrote:
> From: Vincent Mailhol
> > Sent: 02 December 2024 17:33
> >
> > __builtin_constant_p() is known for not always being able to produce
> > constant expression [1] which led to the introduction of
> > __is_constexpr() [2]. Because of its dependency on
> > __builtin_constant_p(), statically_true() suffers from the same
> > issues.
>
> No, they are testing different things.

OK, I will remove this paragraph.

> > For example:
> >
> >   void foo(int a)
> >   {
> >        /* fail on GCC */
> >       BUILD_BUG_ON_ZERO(statically_true(a));
> >
> >        /* fail on both clang and GCC */
> >       static char arr[statically_true(a) ? 1 : 2];
> >   }
> >
> > Define a new is_const_true() and is_const_false() pair of macros
> > which, by making use of __is_const_zero(), always produces a constant
> > expression.
> >
> > Note that is_const_false() can not be directly defined as an alias to
> > __is_const_zero(). Otherwise, it could yield some false positives on
> > huge numbers because of a lost of precision when doing the (long) cast
> > in __is_const_zero(). Example:
> >
> >   is_const_false((u128)ULONG_MAX << BITS_PER_LONG)
> >
> > Furthermore, using the ! operator like this:
> >
> >   #define is_const_true(x) __is_const_zero(!(x))
> >   #define is_const_false(x) __is_const_zero(!!(x))
> >
> > would yield a -Wint-in-bool-context compiler warning if the argument
> > is not a boolean. Use the == and != operators instead.
> >
> > It should be noted that statically_true/false() are the only ones
> > capable of folding tautologic expressions in which at least one on the
> > operands is not a constant expression. For example:
> >
> >   statically_true(true || var)
> >   statically_true(var == var)
> >   statically_false(var * 0)
> >   statically_false(var * 8 % 4)
> >
> > always evaluate to true, whereas all of these would be false under
> > is_const_true/false() if var is not a constant expression [3].
> >
> > For this reason, usage of const_true/false() should be the exception.
> > Reflect in the documentation that const_true() is less powerful and
> > that statically_true() is the overall preferred solution.
> >
> > [1] __builtin_constant_p cannot resolve to const when optimizing
> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449
> >
> > [2] commit 3c8ba0d61d04 ("kernel.h: Retain constant expression output for max()/min()")
> > Link: https://git.kernel.org/torvalds/c/3c8ba0d61d04
> >
> > [3] https://godbolt.org/z/E4r7EaxW9

D'oh, I used some old versions of the macros in that link. The link
will be updated to this in v2:

  https://godbolt.org/z/E4r7EaxW9

> > Signed-off-by: Vincent Mailhol <mailhol.vincent at wanadoo.fr>
> > ---
> >  include/linux/compiler.h | 23 +++++++++++++++++++++++
> >  1 file changed, 23 insertions(+)
> >
> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index 30ce06df4153cfdc0fad9bc7bffab9097f8b0450..165aa5b9bc484376087a130a1ac1f3edb50c983d 100644
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -357,6 +357,29 @@ static inline void *offset_to_ptr(const int *off)
> >   */
> >  #define is_const(x) __is_const_zero(0 * (x))
> >
> > +/*
> > + * Similar to statically_true() but produces a constant expression
>
> No.
> It tests whether a value is a 'constant integer expression' and
> the result is a 'constant integer expression'.
> statically_true() checks for the value being a 'compile time constant'.

I still would argue that ’constant integer expressions’ and ’compile
time constants’ are *similar*. Not the same, agreed, but not
drastically different either. I picked the term *similar* for that
reason.

> Most code really doesn't care, it all got added to min() so that
> a very few places could do:
>         char foo[min(16, sizeof (type))];
> without triggering the 'variable length array' warning.
> But that just bloated everywhere else and (IIRC) Linus replaced
> them with a MIN() that was just an expression.

What about:

  Return an integer constant expression while evaluating if the
  argument is a true (non zero) integer constant expression.



> > + *
> > + * To be used in conjunction with macros, such as BUILD_BUG_ON_ZERO(),
> > + * which require their input to be a constant expression and for which
> > + * statically_true() would otherwise fail.
>
> Use a different BUILD_BUG macro instead.
> Look at the current definition of min().

Do you mean BUILD_BUG_ON_MSG()? That one, at the end, relies on the
error attribute:

  https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-error-function-attribute

And the error attribute logic relies on compiler optimization. So
BUILD_BUG_ON_MSG() is not a valid example here because it does not
require its argument to be an integer constant expression. It works
well with other compile time constants.

Another valid example would be _Static_assert() but as a matter of
fact, it is more common to use __is_constexpr() together with
BUILD_BUG_ON_ZERO() than it is with _Static_assert(). So I think that
BUILD_BUG_ON_ZERO() is best here.


Yours sincerely,
Vincent Mailhol