Hi Martin,
Yes - I believe something more "lightweight" than #pragma is needed.
What I've observed is that it's very "cumbersome", to write
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wvla"
double a[len] ;
#pragma GCC diagnostic push
Technically, can be "compacted" via #define - but then it's becoming
something that it's not "C" any more: e .g.
int foo(int how_many) {
DECLARE_VLA(ddd, double, how_many) ; // Compact but NOT C
int x = 5 ;
}
---
Regarding "vla-larger-than"
-Wvla-larger-than= is a compile-time, size/range-analysis based
warning, so it is very useful for catching potentially large VLAs. I
see it as addressing a different concern than -Wvla.
My goal here is not primarily to allow “small VLAs”, but to make the
presence of a VLA an explicit, local opt-in in codebases that
otherwise keep -Wvla enabled (often as -Werror=vla). Even bounded VLAs
are still a “power-tool” in terms of stack allocation, lifetime, and
portability considerations, and many projects prefer that their use be
visible and deliberate.
-Wvla-larger-than= helps with size risk, but it does not provide:
* a policy mechanism to require explicit acknowledgment of any VLA, or
* a readable, declaration-local marker of intentional use.
So I see the two as complementary:
* -Wvla → policy: VLAs discouraged by default
* [[gnu::allow_vla]] → local, explicit opt-in for specific cases
* -Wvla-larger-than= → safety net for large allocations
I’m intentionally proposing that the attribute suppress only -Wvla,
not -Wvla-larger-than=.
Thanks for the questions/comments — happy to adjust scope if you think
this should be framed differently.
Yair
On Mon, Feb 23, 2026 at 2:44 PM Martin Uecker <[email protected]> wrote:
>
> Am Montag, dem 23.02.2026 um 14:30 -0500 schrieb Yair Lenga via Gcc:
> > 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.
> > -
> >
> > Not a general “suppress any warning” attribute.
> > -
> >
> > C-only (no C++ support requested).
> >
> > Scope restrictions (to reduce ambiguity)
> >
> > I suggest restricting the attribute to:
> >
> > -
> >
> > object declarations with automatic storage duration (typical VLA usage),
> > and
> > -
> >
> > only affecting the -Wvla diagnostic for that declaration.
> >
> > (If maintainers prefer broader applicability, I can adjust, but I’d like to
> > start minimal.)
> > Implementation sketch
> >
> > My assumption is:
> >
> > -
> >
> > parse/attach the attribute to the relevant DECL (e.g., DECL_ATTRIBUTES),
> > -
> >
> > and gate the -Wvla diagnostic emission on “decl has allow_vla”.
> >
> > I can provide a patch + tests once there is agreement on direction and
> > naming.
> > Questions
> >
> > 1.
> >
> > Is a dedicated C-only GNU attribute acceptable for this purpose, vs.
> > insisting on #pragma GCC diagnostic?
>
> I assume you want something more lightweight than
>
> #pragma GCC diagnostic ignored "-Wvla" ?
>
> > 2.
> >
> > Preferred spelling/name: allow_vla, vla_ok, etc.?
> > 3.
> >
> > I intentionally propose -Wvla suppression only; I’m not requesting
> > suppression of -Wvla-larger-than=.
>
> What I do not fully understand is why -Wvla-larger-than= does not
> do what you want already if you are interested in small bounded VLAs?
>
>
>
> Martin
>
>
> >
> > Thanks,
> > Yair
