Re: [PATCH] powerpc: mpc5200: Remove VLA usage

[Michael Ellerman <mpe@ellerman.id.au>]

Segher Boessenkool <segher@kernel.crashing.org> writes:
> On Mon, Jul 02, 2018 at 11:33:32AM +1000, Michael Ellerman wrote:
>> What if we write it:
>> 
>>        char saved_0x500[0x600 - 0x500];
>> 
>> Hopefully the compiler is smart enough not to generate a VLA for that :)
>
> It is a VLA if the array size is not an integer constant expression.  This
> is defined by C; the compiler has nothing to do with it.  0x600-0x500 is
> an integer constant expression, so this is not a VLA.

Thanks.

That wasn't meant as a dig at GCC. Kees had an epic struggle with the
kernel's min/max() macros which were causing expressions that looked
like they should be constant to generate VLAs.

> But if you meant if GCC will ever do a dynamic stack allocation for a fixed
> size local variable: yes indeed, I hope not!

Hey that would be cool, just-in-time local variable allocation :)

cheers