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

[Andrew Pinski <pinskia at gmail dot com>]

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