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Re: gcc warning in my trace_benchmark() code

To: Steven Rostedt <rostedt@goodmis.org>
Subject: Re: gcc warning in my trace_benchmark() code
From: David Daney <ddaney.cavm@gmail.com>
Date: Thu, 05 Jun 2014 11:44:45 -0700
Cc: Ralf Baechle <ralf@linux-mips.org>, linux-mips@linux-mips.org
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On 06/05/2014 10:35 AM, Steven Rostedt wrote:
On Thu, 05 Jun 2014 10:12:16 -0700
David Daney <ddaney.cavm@gmail.com> wrote:

On 06/05/2014 09:12 AM, Steven Rostedt wrote:
I'm going through some of the warnings that Fengguang Wu's test bot has
discovered, and one of them is from a MIPS allmodconfig build.

https://lists.01.org/pipermail/kbuild-all/2014-May/004751.html

     kernel/trace/trace_benchmark.c: In function 'trace_do_benchmark':
kernel/trace/trace_benchmark.c:84:3: warning: comparison of distinct pointer 
types lacks a cast [enabled by default]
kernel/trace/trace_benchmark.c:85:3: warning: comparison of distinct pointer 
types lacks a cast [enabled by default]
     kernel/trace/trace_benchmark.c:38:6: warning: unused variable 'seedsq' 
[-Wunused-variable]

vim +84 kernel/trace/trace_benchmark.c

      78                if (bm_cnt > 1) {
      79                        /*
      80                         * Apply Welford's method to calculate standard 
deviation:
      81                         * s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - 
(\Sum x_i)^2)
      82                         */
      83                        stddev = (u64)bm_cnt * bm_totalsq - bm_total * 
bm_total;
    > 84                     do_div(stddev, bm_cnt);
    > 85                     do_div(stddev, bm_cnt - 1);
      86                } else
      87                        stddev = 0;
      88        



Is there something special with do_div in mips that I should be aware
of?

Yes.  MIPS is using the implementation in asm-generic/div64.h,  which
per the comments in that file has a useless pointer compare to find just
this type of issue.

You mean this comment?

/* The unnecessary pointer compare is there
  * to check for type safety (n must be 64bit)
  */


Yes.

But stddev is s64. Ah, but the compare is:

(void)(((typeof((n)) *)0) == ((uint64_t *)0));

so it's complaining about a signed verses unsigned compare, not length.
I think I can ignore this warning then.

The pedant in me thinks that you should fix your code if using do_div() on a signed object is undefined. But if you aren't planning on merging the code, then it probably doesn't matter.


Thoughts?

I think I will have lunch now...

David Daney



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