On 5/6/24 2:07 AM, Ilpo Järvinen wrote:
On Fri, 3 May 2024, John Hubbard wrote:
On 5/3/24 11:37 AM, Reinette Chatre wrote:
On 5/3/2024 9:52 AM, John Hubbard wrote:
On 5/3/24 1:00 AM, Ilpo Järvinen wrote:
On Thu, 2 May 2024, John Hubbard wrote:
...
diff --git a/tools/testing/selftests/resctrl/mbm_test.c b/tools/testing/selftests/resctrl/mbm_test.c index d67ffa3ec63a..c873793d016d 100644 --- a/tools/testing/selftests/resctrl/mbm_test.c +++ b/tools/testing/selftests/resctrl/mbm_test.c @@ -33,7 +33,7 @@ show_bw_info(unsigned long *bw_imc, unsigned long *bw_resc, size_t span) avg_bw_imc = sum_bw_imc / 4; avg_bw_resc = sum_bw_resc / 4; - avg_diff = (float)labs(avg_bw_resc - avg_bw_imc) / avg_bw_imc; + avg_diff = (float)(avg_bw_resc - avg_bw_imc) / avg_bw_imc; avg_diff_per = (int)(avg_diff * 100); ret = avg_diff_per > MAX_DIFF_PERCENT;
But how are these two cases same after your change when you ended up removing taking the absolute value entirely?
All of the arguments are unsigned integers, so all arithmetic results are interpreted as unsigned, so taking the absolute value of that is always a no-op.
(I see there's a better patch posted already but since there are a few incorrect claims in this discussion, I'll do for the record type of reply.)
This discussion now went to a tangent about the warning. My main point is that logic is not correct after removing labs().
I also disagree with the claim that using labs() on unsigned value is no-op because labs() takes long so unsigned is just forced into signed when calling which is why the warning triggers but it's very misleading warning (see below).
Yes you are correct.
It does not seem as though clang can see when values have been casted. I tried to do so explicitly with a: avg_diff = labs((long)avg_bw_resc - avg_bw_imc) / (float)avg_bw_imc;
The subtraction result will get promoted to an unsigned long, before being passed into labs(3).
But that still triggers: warning: taking the absolute value of unsigned type 'unsigned long' has no effect [-Wabsolute-value]
As expected, yes.
That error message isn't factually correct:
unsigned long a = LONG_MAX; long b; a += 2; b = (long)a; printf("%llu %lli %lli\n", a, b, labs(a));Prints (at least when built with gcc):
9223372036854775809 -9223372036854775807 9223372036854775807
labs(LONG_MAX + 1) won't work though since it's not positively presentable with long and the value is left untouched.
Thanks for setting the detailed record straight! :)
thanks,