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@xxxxxxxxxx> > --- > 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 > > - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)