mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/
synced 2026-11-10 20:48:12 +00:00
No description
- C 96.8%
- Assembly 0.9%
- Rust 0.7%
- Shell 0.6%
- Python 0.5%
- Other 0.3%
4f3d1be4c2
__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
tautological 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() 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
|
History
|
|
|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.