[COMMITTED] Fix string/tst-xbzero-opt if build with gcc head.

[Stefan Liebler]

On 07/26/2018 03:33 PM, Carlos O'Donell wrote:
> On 07/16/2018 07:05 AM, Stefan Liebler wrote:
>> On 07/12/2018 06:42 PM, Zack Weinberg wrote:
>>> On Thu, Jul 12, 2018 at 10:22 AM, Stefan Liebler <stli@linux.ibm.com> wrote:
>>>> Fix string/tst-xbzero-opt is build with gcc head.
>>> ...
>>>> In setup_no_clear / setup_ordinary_clear, GCC is omitting the memcpy loop in
>>>> prepare_test_buffer. Thus count_test_patterns does not find any of the
>>>> test_pattern.
>>>>
>>>> This patch introduces a compiler barrier just after filling the buffer.
>>>
>>> I think I understand why the call to swapcontext in
>>> prepare_test_buffer is not a sufficient compiler barrier, but I am not
>>> a fan of asm volatile ("" ::: "memory"), because I fully expect some
>>> future compiler to decide that there are no actual assembly
>>> instructions being inserted so the statement can be completely
>>> ignored.  I would prefer us to find some kind of construct that
>>> actually does make externally-visible side effects depend on the
>>> contents of 'buf' in terms of the C abstract machine.
>>>
>>> zw
>>>
>>
>> Okay. Then here is a new version of the patch without the empty asm.
>> If build with GCC head on s390x, setup_no_clear is now filling the
>> buffer and setup_ordinary_clear is just a tail call to setup_no_clear
>> (same behaviour as before).
> 
> Thanks for fixing this.
> 
>> commit a42ce495317df04f1abdc3670c1fcbb68f329c3d
>> Author: Stefan Liebler <stli@linux.ibm.com>
>> Date:   Fri Jul 13 15:58:48 2018 +0200
>>
>>      Fix string/tst-xbzero-opt if build with gcc head.
>>      
>>      On s390x, the test string/tst-xbzero-opt is failing if build with gcc head:
>>      FAIL: no clear/prepare: expected 32 got 0
>>      FAIL: no clear/test: expected some got 0
>>      FAIL: ordinary clear/prepare: expected 32 got 0
>>      INFO: ordinary clear/test: found 0 patterns (memset not eliminated)
>>      PASS: explicit clear/prepare: expected 32 got 32
>>      PASS: explicit clear/test: expected 0 got 0
>>      
>>      In setup_no_clear / setup_ordinary_clear, GCC is omitting the memcpy loop
>>      in prepare_test_buffer. Thus count_test_patterns does not find any of the
>>      test_pattern.
>>      
>>      This patch calls use_test_buffer in order to force the compiler to really copy
>>      the pattern to buf.
> 
> OK.
> 
>>      
>>      ChangeLog:
>>      
>>              * string/tst-xbzero-opt.c (use_test_buffer): New function.
>>               (prepare_test_buffer): Call use_test_buffer as compiler barrier.
> 
> OK for 2.28. Consider noclone also.
> 
> Reviewed-by: Carlos O'Donell <carlos@redhat.com>
> 

Okay. I've just committed this patch with noclone.

Thanks
Stefan

>>
>> diff --git a/string/tst-xbzero-opt.c b/string/tst-xbzero-opt.c
>> index cf7041f37a..b00fafd237 100644
>> --- a/string/tst-xbzero-opt.c
>> +++ b/string/tst-xbzero-opt.c
>> @@ -97,6 +97,17 @@ static const unsigned char test_pattern[16] =
>>   
>>   static ucontext_t uc_main, uc_co;
>>   
>> +static __attribute__ ((noinline)) int
> 
> Suggest: Add noclone.
> 
> As optimization barriers we have also used "noclone" here
> e.g. malloc/tst-mallocstate.c (my_free).
> 
> I can't see a case where a specialization of this function avoids
> the copy, but it might be possible? Just for safety's sake use noclone
> too.
> 
>> +use_test_buffer (unsigned char *buf)
>> +{
>> +  unsigned int sum = 0;
>> +
>> +  for (unsigned int i = 0; i < PATTERN_REPS; i++)
>> +    sum += buf[i * PATTERN_SIZE];
>> +
>> +  return (sum == 2 * PATTERN_REPS) ? 0 : 1;
>> +}
> 
> OK.
> 
>> +
>>   /* Always check the test buffer immediately after filling it; this
>>      makes externally visible side effects depend on the buffer existing
>>      and having been filled in.  */
>> @@ -108,6 +119,10 @@ prepare_test_buffer (unsigned char *buf)
>>   
>>     if (swapcontext (&uc_co, &uc_main))
>>       abort ();
>> +
>> +  /* Force the compiler to really copy the pattern to buf.  */
>> +  if (use_test_buffer (buf))
>> +    abort ();
> 
> OK.
> 
>>   }
>>   
>>   static void
>