__builtin_constant_p() is known for not always being able to produce
constant expression [1] which lead 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 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 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 test.
Declare a new _statically_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() 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 return false.
For this reason, usage of _statically_true() be should 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@xxxxxxxxxx>
---
Bonuses:
- above examples, and a bit more:
https://godbolt.org/z/zzqM1ajPj
- a proof that statically_true() does better constant folding than _statically_true()
https://godbolt.org/z/vK6KK4hMG
---
include/linux/compiler.h | 14 ++++++++++++++
1 file changed, 14 insertions(+)
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 4d4e23b6e3e7..c76db8b50202 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -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 subcomponent 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.
--
2.45.2
