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

[Stefan Liebler <stli at linux dot ibm dot com>]

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

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