On 23/02/2026 20:30, Yair Lenga via Gcc wrote:
Hi GCC maintainers,
I’d like feedback on a small, C-only attribute proposal to improve
practicality of using -Wvla in large codebases.
Motivation
Projects often enable -Wvla (sometimes with -Werror=vla) to discourage
unbounded VLAs. However, there are occasional VLAs that are intentionally
bounded/validated (e.g., small scratch buffers sized from known limits).
Today, suppressing -Wvla locally requires #pragma GCC diagnostic
push/ignored/pop, which is verbose and harder to keep correct during
refactoring.
I’m looking for a declaration-scoped, readable opt-out similar in spirit to
[[fallthrough]]: keep the warning enabled globally, but allow a
clearly-marked exception at a specific declaration.
Proposal (C only)
Introduce a GNU C attribute:
[[gnu::allow_vla]] char tmp[n];
(Spelling is flexible; I mainly want the functionality. I’m fine with
__attribute__((allow_vla)) as well, but [[gnu::...]] is pleasant in C23
style.)
Semantics
When -Wvla is enabled, a VLA declaration annotated with [[gnu::allow_vla]]
does *not* emit the -Wvla warning for that declaration.
Non-goals:
-
No change to VLA semantics, ABI, or code generation.
OK.
-
Not a general “suppress any warning” attribute.
-
Why not?
C-only (no C++ support requested).
Why?
How about introducing new attributes:
[[gnu::diagnostic_ignored("-Wvla")]]
[[gnu::diagnostic_warning("-Wvla")]]
[[gnu::diagnostic_error("-Wvla")]]
such that writing
[[gnu::diagnostic_ignored("-Wvla")]] char tmp[n];
would have the same effect as
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wvla"
char tmp[n];
#pragma GCC diagnostic pop
?
That is, rather than making a single attribute for a single warning,
make a general attribute (or three general attributes) that work for any
warning. The potential benefits would be vastly greater, even though it
might involve a bit more work in implementation. It would certainly be
less work than adding (and documenting!) similar attributes for a range
of other warnings. From a quick check in a recent project, I've used
the push/ignored/pop pattern for -Wpadded, -Wcast-align and
-Wfloat-equal. And if it is of interest to have attributes for
temporarily disabling more than one warning type, having a consistent
naming that follows the warning flag is hugely better than ad-hoc names
like "allow_vla", "might_be_padded", "cast_unaligned",
"compare_float_equal".
For just this on-off case, I am not sure I see how useful such an
attribute would be compared to just defining a macro :
#define ALLOW_VLA(decl) \
_Pragma("GCC diagnostic push") \
_Pragma("GCC diagnostic ignored \"-Wvla\"") \
decl; \
_Pragma("GCC diagnostic pop")
And then writing "ALLOW_VLA(char tmp[n]);"
David
