[tarantool-patches] Re: [PATCH 1/3] Fix: prevent guard-breaker optimization
Alex Khatskevich
avkhatskevich at tarantool.org
Thu Oct 11 19:02:23 MSK 2018
On 10.10.2018 17:26, Alexander Turenko wrote:
> 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.
Discussed verbally.
Added a comment near volatile keyword.
>> memset(block, 0xff, 2048);
>> sum += block[block[4]];
>> ptrdiff_t stack_diff = ptr > block ? ptr - block : block - ptr;
>> --
>> 2.14.1
>>
>>
New diff:
commit d7acdc09aa471ea15b3821bab205d8fcdb9280c1
Author: AKhatskevich <avkhatskevich at tarantool.org>
Date: Tue Aug 7 16:47:39 2018 +0300
Fix: prevent guard-breaker optimization
In case of very aggressive optimizations the compiler can
optimize guard-breaker function away and the `unit/guard`
test would fail.
diff --git a/test/unit/guard.cc b/test/unit/guard.cc
index 231b44c7d..3d42fee31 100644
--- a/test/unit/guard.cc
+++ b/test/unit/guard.cc
@@ -13,7 +13,11 @@ static int __attribute__((noinline))
stack_break_f(char *ptr)
{
char block[2048];
- char sum = 0;
+ /*
+ * Make sum volatile to prevent a compiler from
+ * optimizing away call to this function.
+ */
+ volatile char sum = 0;
memset(block, 0xff, 2048);
sum += block[block[4]];
ptrdiff_t stack_diff = ptr > block ? ptr - block : block - ptr;
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