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Re: gcc warning in my trace_benchmark() code

To: David Daney <ddaney.cavm@gmail.com>
Subject: Re: gcc warning in my trace_benchmark() code
From: Steven Rostedt <rostedt@goodmis.org>
Date: Thu, 5 Jun 2014 13:35:00 -0400
Cc: Ralf Baechle <ralf@linux-mips.org>, linux-mips@linux-mips.org
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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)
 */

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.

Thoughts?

-- Steve


> 
> 
> Ralf:  As a side note, while looking at arch/mips/include/asm/div64.h, I 
> saw that the implementation of __div64_32 in that file will be unused, 
> and is also completely broken due to the first parameter never being used.
> 
> David Daney
> 


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