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

[David Laight <David.Laight@ACULAB.COM>]

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.


> 
> 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
> 
> Signed-off-by: Vincent Mailhol <mailhol.vincent@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'.

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.

> + *
> + * 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().

	David

> + *
> + * This is a trade-off: is_const_true() requires all its operands to
> + * be compile time constants. Else, it would always returns false even
> + * on the most trivial cases like:
> + *
> + *   true || non_const_expr
> + *
> + * On the opposite, statically_true() is able to fold more complex
> + * tautologies and will return true on expressions such as:
> + *
> + *   !(non_const_expr * 8 % 4)
> + *
> + * For the general case, statically_true() is better.
> + */
> +#define is_const_true(x) __is_const_zero((x) == 0)
> +#define is_const_false(x) __is_const_zero((x) != 0)
> +
>  /*
>   * This is needed in functions which generate the stack canary, see
>   * arch/x86/kernel/smpboot.c::start_secondary() for an example.
> 
> --
> 2.45.2
> 
> 

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