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

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

[Stefan Liebler]

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Fix string/tst-xbzero-opt is 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 introduces a compiler barrier just after filling the buffer.

If okay, shall this be committed before or after the release?

Bye
Stefan

---

ChangeLog:

	* string/tst-xbzero-opt.c (prepare_test_buffer):
	Add compiler barrier.

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commit 65f9b078c5053faa93e1f572282463685a869864
Author: Stefan Liebler <stli@linux.ibm.com>
Date:   Thu Jul 12 16:07:26 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 introduces a compiler barrier just after filling the buffer
    and the filling of the test_pattern is not omitted.
    
    ChangeLog:
    
            * string/tst-xbzero-opt.c (prepare_test_buffer): Add compiler barrier.

diff --git a/string/tst-xbzero-opt.c b/string/tst-xbzero-opt.c
index cf7041f37a..4c2be0c197 100644
--- a/string/tst-xbzero-opt.c
+++ b/string/tst-xbzero-opt.c
@@ -106,6 +106,9 @@ prepare_test_buffer (unsigned char *buf)
   for (unsigned int i = 0; i < PATTERN_REPS; i++)
     memcpy (buf + i*PATTERN_SIZE, test_pattern, PATTERN_SIZE);
 
+  /* Force the compiler to really copy the pattern to buf.  */
+  __asm__ __volatile__ ("" ::: "memory");
+
   if (swapcontext (&uc_co, &uc_main))
     abort ();
 }

Re: Fix string/tst-xbzero-opt if build with gcc head.

[Zack Weinberg]

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

Re: Fix string/tst-xbzero-opt if build with gcc head.

[Stefan Liebler]

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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).

Is this C construct okay?

Bye.
Stefan

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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.
    
    ChangeLog:
    
            * string/tst-xbzero-opt.c (use_test_buffer): New function.
             (prepare_test_buffer): Call use_test_buffer as compiler barrier.

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
+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;
+}
+
 /* 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 ();
 }
 
 static void

Re: Fix string/tst-xbzero-opt if build with gcc head.

[Zack Weinberg]

On Mon, Jul 16, 2018 at 7:05 AM, Stefan Liebler <stli@linux.ibm.com> wrote:
> On 07/12/2018 06:42 PM, Zack Weinberg wrote:
>> 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.
>>
>
> 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).

Yes, this change is OK.

zw

Re: Fix string/tst-xbzero-opt if build with gcc head.

[Stefan Liebler]

On 07/16/2018 02:36 PM, Zack Weinberg wrote:
> On Mon, Jul 16, 2018 at 7:05 AM, Stefan Liebler <stli@linux.ibm.com> wrote:
>> On 07/12/2018 06:42 PM, Zack Weinberg wrote:
>>> 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.
>>>
>>
>> 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).
> 
> Yes, this change is OK.
> 
> zw
> 

Okay. Thank you.
Shall I commit this test-fix before or after the release?

Bye.
Stefan

Re: Fix string/tst-xbzero-opt if build with gcc head.

[Zack Weinberg]

On Mon, Jul 16, 2018 at 9:13 AM, Stefan Liebler <stli@linux.ibm.com> wrote:
> On 07/16/2018 02:36 PM, Zack Weinberg wrote:
>> On Mon, Jul 16, 2018 at 7:05 AM, Stefan Liebler <stli@linux.ibm.com>
>> wrote:
>>> 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).
>>
>>
>> Yes, this change is OK.
>
> Okay. Thank you.
> Shall I commit this test-fix before or after the release?

Carlos has the last word but we usually accept bugfixes to test cases
right up till the hard freeze.

zw

Re: Fix string/tst-xbzero-opt if build with gcc head.

[Stefan Liebler]

On 07/16/2018 03:18 PM, Zack Weinberg wrote:
> On Mon, Jul 16, 2018 at 9:13 AM, Stefan Liebler <stli@linux.ibm.com> wrote:
>> On 07/16/2018 02:36 PM, Zack Weinberg wrote:
>>> On Mon, Jul 16, 2018 at 7:05 AM, Stefan Liebler <stli@linux.ibm.com>
>>> wrote:
>>>> 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).
>>>
>>>
>>> Yes, this change is OK.
>>
>> Okay. Thank you.
>> Shall I commit this test-fix before or after the release?
> 
> Carlos has the last word but we usually accept bugfixes to test cases
> right up till the hard freeze.
> 
> zw
> 

Carlos,

what about this bugfix for this test?
Can I commit it now or shall I wait commit it after the release?

Bye
Stefan

Re: Fix string/tst-xbzero-opt if build with gcc head.

[Carlos O'Donell]

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>

> 
> 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

Re: [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
>