On Fri, Mar 16, 2018 at 10:44 AM, David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> I looked at the generated code for one of the constant sized VLA that
> the compiler barfed at.
> It seemed to subtract constants from %sp separately for the VLA.
> So it looks like the compiler treats them as VLA even though it
> knows the size.
> That is probably missing optimisation.
Looking at the code is definitely an option.
In fact, instead of depending on -Wvla, we could just make 'objtool'
warn about real variable-sized stack frames.
That said, if that "sizeof()" trick of Al's actually works with older
gcc versions too (it *should*, but it's not like
__builtin_choose_expr() and __builtin_constant_p() have well-defined
rules in the standard), that may just be the solution.
And if gcc ends up generating bad code for those "constant sized vlas"
anyway, then -Wvla would effectively warn about that code generation
problem.
Linus
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