On Wed, Nov 13, 2024 at 10:53:55AM -0800, Yury Norov wrote:
> On Thu, Nov 14, 2024 at 02:18:32AM +0900, Vincent Mailhol wrote:
> > __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.
> >
> > 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];
> > }
> >
> > For the same reasons why __is_constexpr() was created to cover
> > __builtin_constant_p() edge cases, __is_constexpr() can be used to
> > resolve statically_true() limitations.
> >
> > Note that, somehow, GCC is not always able to fold this:
> >
> > __is_constexpr(x) && (x)
> >
> > It is OK in BUILD_BUG_ON_ZERO() but not in array declarations nor in
> > static_assert():
> >
> > void bar(int a)
> > {
> > /* success */
> > BUILD_BUG_ON_ZERO(__is_constexpr(a) && (a));
> >
> > /* fail on GCC */
> > static char arr[__is_constexpr(a) && (a) ? 1 : 2];
> >
> > /* fail on GCC */
> > static_assert(__is_constexpr(a) && (a));
> > }
> >
> > Encapsulating the expression in a __builtin_choose_expr() switch
> > resolves all these failed tests.
> >
> > Define a new const_true() macro which, by making use of the
> > __builtin_choose_expr() and __is_constexpr(x) combo, always produces a
> > constant expression.
> >
> > It should be noted that statically_true() is the only one able to fold
> > 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_true(var * 0 + 1)
> > statically_true(!(var * 8 % 4))
> >
> > always evaluates to true, whereas all of these would be false under
> > const_true() if var is not a constant expression [3].
> >
> > For this reason, usage of const_true() be should 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/c61PMxqbK
> >
> > Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
>
> For the series:
>
> Reviewed-by: Yury Norov <yury.norov@xxxxxxxxx>
>
> If no objections, I'll move it with my tree.
This is already in my branch, but there was a discussion after I pulled
it. Can you guys tell me what is your conclusion on that? Should I
keep it in the branch, or drop?
Thanks,
Yury
