Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 16 Mar 2018 13:25:26 -0700 | Subject | Re: [PATCH v5 0/2] Remove false-positive VLAs when using max() |
| |
On Fri, Mar 16, 2018 at 10:44 AM, David Laight <David.Laight@aculab.com> 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
| |