Re: gcc warning in my trace_benchmark() code

[David Daney <ddaney.cavm@gmail.com>]

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.


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