[tarantool-patches] Re: [PATCH 1/3] Fix: prevent guard-breaker optimization

[Alexander Turenko]

On Wed, Aug 08, 2018 at 02:10:01PM +0300, AKhatskevich wrote:
> In case of very aggressive optimizations the compiler can
> optimize guard-breaker function away and the `unit/guard`
> test would fail.

I think it is good to mention the specific compiler options (and a
certain compiler you are using to reproduce it) to give a user an idea
when things are going wrong and so what is the fix does.

Is it due to discarding the noinline attribute in case of LTO on gcc? So
'volatile' prevents inlining the function? I just guessing here, but I
think the commit message should clarify such things if possible.

> ---
>  test/unit/guard.cc | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/test/unit/guard.cc b/test/unit/guard.cc
> index 231b44c7d..2082dfd48 100644
> --- a/test/unit/guard.cc
> +++ b/test/unit/guard.cc
> @@ -13,7 +13,7 @@ static int __attribute__((noinline))
>  stack_break_f(char *ptr)
>  {
>  	char block[2048];
> -	char sum = 0;
> +	volatile char sum = 0;

I think a reason of using 'volatile' keyword should be always properly
commented in the code, because it always fix some unobvious compiler
behaviour.

>  	memset(block, 0xff, 2048);
>  	sum += block[block[4]];
>  	ptrdiff_t stack_diff = ptr > block ? ptr - block : block - ptr;
> -- 
> 2.14.1
> 
>