Re: Frame pointer, bug or feature? (x86)

[Hendrik Greving <hendrik dot greving dot intel at gmail dot com>]

Interesting, I just read up on it and I didn't know that. Thanks. Is
it correct to say though that it is a missing optimization and
frame_pointer_needed shouldn't evaluate to true?

On Mon, Nov 18, 2013 at 10:55 AM, Andrew Pinski <pinskia@gmail.com> wrote:
> On Mon, Nov 18, 2013 at 10:47 AM, Hendrik Greving
> <hendrik.greving.intel@gmail.com> wrote:
>> What's the difference in the C vs. the C++ spec that makes it a VLA in GNU-C?
>
>
> max in C++ is considered an integer constant expression while in C it
> is just an expression.
>
> Thanks,
> Andrew Pinski
>
>>
>> On Fri, Nov 15, 2013 at 10:07 AM, Andrew Pinski <pinskia@gmail.com> wrote:
>>> On Fri, Nov 15, 2013 at 9:31 AM, Hendrik Greving
>>> <hendrik.greving.intel@gmail.com> wrote:
>>>> In the below test case, "CASE_A" actually uses a frame pointer, while
>>>> !CASE_A doesn't. I can't imagine this is a feature, this is a bug,
>>>> isn't it? Is there any reason the compiler couldn't know that
>>>> loop_blocks never needs a dynamic stack size?
>>>
>>>
>>> Both a feature and a bug.  In the CASE_A case (with GNU C) it is a VLA
>>> while in the !CASE_A case (or in either case with C++), it is a normal
>>> array definition.  The compiler could have converted the VLA to a
>>> normal array but does not depending on the size of the array.
>>>
>>> Thanks,
>>> Andrew Pinski
>>>
>>>>
>>>> #include <stdio.h>
>>>> #include <stdlib.h>
>>>>
>>>> #define MY_DEFINE 100
>>>> #define CASE_A 1
>>>>
>>>> extern init(int (*a)[]);
>>>>
>>>> int
>>>> foo()
>>>> {
>>>> #if CASE_A
>>>>     const int max = MY_DEFINE * 2;
>>>>     int loop_blocks[max];
>>>> #else
>>>>     int loop_blocks[MY_DEFINE * 2];
>>>> #endif
>>>>     init(&loop_blocks);
>>>>     return loop_blocks[5];
>>>> }
>>>>
>>>> int
>>>> main()
>>>> {
>>>>     int i = foo();
>>>>     printf("is is %d\n", i);
>>>> }
>>>>
>>>> Thanks,
>>>> Hendrik Greving