docs/coding_style: Clarify use of GCC extensions

This patch adds a section to the coding style that explicitly clarifies
the use of GCC extensions in coreboot (which has been long-standing
practice anyway), and expressly allows their use.

See the mailing list discussion for more details:
https://mail.coreboot.org/hyperkitty/list/coreboot@coreboot.org/thread/3C2QWAZ5RJ6ME5KXMEOGB5GW62UTXCLS/

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: I0d0eb90d6729fefeb131cdd573ad51f1884afe11
Reviewed-on: https://review.coreboot.org/c/coreboot/+/63660
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Krystian Hebel <krystian.hebel@3mdeb.com>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>
This commit is contained in:
Julius Werner 2022-04-15 14:26:33 -07:00 committed by Felix Held
parent 9cb5dcb40c
commit 5784ab34d4
1 changed files with 35 additions and 0 deletions

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@ -960,6 +960,41 @@ asm ("magic %reg1, #42nt"
: /* outputs */ : /* inputs */ : /* clobbers */); : /* outputs */ : /* inputs */ : /* clobbers */);
``` ```
GCC extensions
--------------
GCC is the only officially-supported compiler for coreboot, and a
variety of its C language extensions are heavily used throughout the
code base. There have been occasional attempts to add clang as a second
compiler option, which is generally compatible to the same language
extensions that have been long-established by GCC.
Some GCC extensions (e.g. inline assembly) are basically required for
proper firmware development. Others enable more safe or flexible
coding patterns than can be expressed with standard C (e.g. statement
expressions and `typeof()` to avoid double evaluation in macros like
`MAX()`). Yet others just add some simple convenience and reduce
boilerplate (e.g. `void *` arithmetic).
Since some GCC extensions are necessary either way, there is no gain
from avoiding other GCC extensions elsewhere. The use of all official
GCC extensions is expressly allowed within coreboot. In cases where an
extension can be replaced by a 100% equivalent C standard feature with
no extra boilerplate or loss of readability, the C standard feature
should be preferred (this usually only happens when GCC retains an
older pre-standardization extension for backwards compatibility, e.g.
the old pre-C99 syntax for designated initializers). But if there is
any advantage offered by the GCC extension (e.g. using GCC zero-length
arrays instead of C99 variable-length arrays because they don't inhibit
`sizeof()`), there is no reason to deprive ourselves of that, and "this
is not C standard compliant" should not be a reason to argue against
its use in reviews.
This rule only applies to explicit GCC extensions listed in the
"Extensions to the C Language Family" section of the GCC manual. Code
should never rely on incidental GCC translation behavior that is not
explicitly documented as a feature and could change at any moment.
References References
---------- ----------