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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: Ralf Baechle <ralf@linux-mips.org>
Date: Thu, 5 Jun 2014 23:07:19 +0200
Cc: Steven Rostedt <rostedt@goodmis.org>, linux-mips@linux-mips.org, "Maciej W. Rozycki" <macro@linux-mips.org>
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On Thu, Jun 05, 2014 at 10:12:16AM -0700, David Daney wrote:

> >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.

And it's not the only warning it's picking up.

> 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.

Seems I broke c21004cd5b4cb7d479514d470a62366e8307412c "MIPS: Rewrite
<asm/div64.h> to work with gcc 4.4.0."  Took only five years until
somebody noticed ...

  Ralf

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