@@ -308,6 +308,20 @@ static inline void *offset_to_ptr(const int *off)
*/
#define statically_true(x) (__builtin_constant_p(x) && (x))
+/*
+ * Similar to statically_true() but produces a 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.
+ *
+ * This is a tradeoff: _statically_true() is less efficient at
+ * constant folding and will fail to optimize any expressions in which
+ * at least one of the subcomponents is not constant. For the general
+ * case, statically_true() is better.
+ */
+#define _statically_true(x) __builtin_choose_expr(__is_constexpr(x), x, false)
+
/*
* This is needed in functions which generate the stack canary, see
* arch/x86/kernel/smpboot.c::start_secondary() for an example.
__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 remediate __builtin_constant_p() edge cases, __is_constexpr() can be used to resolve statically_true()'s 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 or 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 examples. Declare a new _statically_true() macro which, by making use of the __builtin_choose_expr() and __is_constexpr() combo, always produces a constant expression. It should be noted that statically_true() still produces better folding: statically_true(!(var * 8 % 8)) always evaluates to true even if var is unknown, whereas _statically_true(!(var * 8 % 8)) fails to fold the expression and returns false. For this reason, usage of _statically_true() should be the exception. Reflect in the documentation that _statically_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()") Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr> --- Bonuses: - above examples (and a bit more) in godbolt: https://godbolt.org/z/GYeEK5d7s - that proof, in godbolt, that statically_true() is bettera at constant folding than _statically_true() https://godbolt.org/z/vK6KK4hMG --- include/linux/compiler.h | 14 ++++++++++++++ 1 file changed, 14 insertions(+)