We recently started to get random failures on the common_autotest ut with clang on Ubuntu 16.04.6. Example: https://travis-ci.com/DPDK/dpdk/jobs/263177424 Wrong rte_log2_u64(0) val 0, expected ffffffff Test Failed The ut passes 0 to log2() to get an expected value. Quoting log2 / log(3) manual: If x is zero, then a pole error occurs, and the functions return -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively. rte_log2_uXX helpers handle 0 as a special value and return 0. Let's have dedicated tests for this case. Fixes: 05c4345ef5c2 ("test: add unit test for integer log2 function") Cc: stable@dpdk.org Signed-off-by: David Marchand <david.marchand@redhat.com> --- app/test/test_common.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) diff --git a/app/test/test_common.c b/app/test/test_common.c index 2b856f8ba5..12bd1cad90 100644 --- a/app/test/test_common.c +++ b/app/test/test_common.c @@ -216,7 +216,19 @@ test_log2(void) const uint32_t max = 0x10000; const uint32_t step = 1; - for (i = 0; i < max; i = i + step) { + compare = rte_log2_u32(0); + if (compare != 0) { + printf("Wrong rte_log2_u32(0) val %x, expected 0\n", compare); + return TEST_FAILED; + } + + compare = rte_log2_u64(0); + if (compare != 0) { + printf("Wrong rte_log2_u64(0) val %x, expected 0\n", compare); + return TEST_FAILED; + } + + for (i = 1; i < max; i = i + step) { uint64_t i64; /* extend range for 64-bit */ -- 2.23.0

```
David Marchand <david.marchand@redhat.com> writes:
> We recently started to get random failures on the common_autotest ut with
> clang on Ubuntu 16.04.6.
>
> Example: https://travis-ci.com/DPDK/dpdk/jobs/263177424
>
> Wrong rte_log2_u64(0) val 0, expected ffffffff
> Test Failed
>
> The ut passes 0 to log2() to get an expected value.
>
> Quoting log2 / log(3) manual:
> If x is zero, then a pole error occurs, and the functions return
> -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.
>
> rte_log2_uXX helpers handle 0 as a special value and return 0.
> Let's have dedicated tests for this case.
>
> Fixes: 05c4345ef5c2 ("test: add unit test for integer log2 function")
> Cc: stable@dpdk.org
>
> Signed-off-by: David Marchand <david.marchand@redhat.com>
> ---
Acked-by: Aaron Conole <aconole@redhat.com>
Somethings that concern me:
1. A log2(0) should probably be an undetermined value, but this
effectively makes log2(0) == log2(1) so that if anyone uses these
for some mathematical work, it will have an exceptional behavior. I
know it's documented from a programmer perspective, but I am all for
documenting the mathematical side effect as well.
2. Why hasn't this been complaining for so long? Or has it and we just
haven't noticed? Were some compiler flags changed recently? (maybe
-funsafe-math was always set or something?)
-Aaron
```

On Wed, Dec 4, 2019 at 10:20 PM Aaron Conole <aconole@redhat.com> wrote: > > David Marchand <david.marchand@redhat.com> writes: > > > We recently started to get random failures on the common_autotest ut with > > clang on Ubuntu 16.04.6. > > > > Example: https://travis-ci.com/DPDK/dpdk/jobs/263177424 > > > > Wrong rte_log2_u64(0) val 0, expected ffffffff > > Test Failed > > > > The ut passes 0 to log2() to get an expected value. > > > > Quoting log2 / log(3) manual: > > If x is zero, then a pole error occurs, and the functions return > > -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively. > > > > rte_log2_uXX helpers handle 0 as a special value and return 0. > > Let's have dedicated tests for this case. > > > > Fixes: 05c4345ef5c2 ("test: add unit test for integer log2 function") > > Cc: stable@dpdk.org > > > > Signed-off-by: David Marchand <david.marchand@redhat.com> > > --- > > Acked-by: Aaron Conole <aconole@redhat.com> > > Somethings that concern me: > > 1. A log2(0) should probably be an undetermined value, but this > effectively makes log2(0) == log2(1) so that if anyone uses these > for some mathematical work, it will have an exceptional behavior. I > know it's documented from a programmer perspective, but I am all for > documenting the mathematical side effect as well. What do you have in mind, adding a big warning in the doxygen "THIS IS NOT REAL MATH STUFF" ? :-) > > 2. Why hasn't this been complaining for so long? Or has it and we just > haven't noticed? Were some compiler flags changed recently? (maybe > -funsafe-math was always set or something?) Yes, I wondered too how we did not see this earlier. Even now, it happens randomly. libc log2(0) is not undefined itself, it returns -infinity. Looking at the test code, ceilf (I would expect ceil) returns -infinity when getting it passed. So I'd say the problem lies in the cast to uint32_t. Both gcc and clang do not seem to bother with standard compilation flags, and the cast gives 0 on my RHEL. #include <stdio.h> #include <inttypes.h> #include <math.h> int main(int argc, char *argv[]) { printf("%lf %f %"PRIu32"\n", log2(0), (float)log2(0), (uint32_t)log2(0)); return 0; } $ ./log2 -inf -inf 0 Now, if I use UBSAN with clang, I get a complaint at runtime: $ ./log2 (/home/dmarchan/log2+0x41dfa5): runtime error: value -inf is outside the range of representable values of type 'unsigned int' -inf -inf 0 Not sure if it explains the random failures, but won't undefined behavior eat your babies? -- David Marchand

David Marchand <david.marchand@redhat.com> writes: > On Wed, Dec 4, 2019 at 10:20 PM Aaron Conole <aconole@redhat.com> wrote: >> >> David Marchand <david.marchand@redhat.com> writes: >> >> > We recently started to get random failures on the common_autotest ut with >> > clang on Ubuntu 16.04.6. >> > >> > Example: https://travis-ci.com/DPDK/dpdk/jobs/263177424 >> > >> > Wrong rte_log2_u64(0) val 0, expected ffffffff >> > Test Failed >> > >> > The ut passes 0 to log2() to get an expected value. >> > >> > Quoting log2 / log(3) manual: >> > If x is zero, then a pole error occurs, and the functions return >> > -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively. >> > >> > rte_log2_uXX helpers handle 0 as a special value and return 0. >> > Let's have dedicated tests for this case. >> > >> > Fixes: 05c4345ef5c2 ("test: add unit test for integer log2 function") >> > Cc: stable@dpdk.org >> > >> > Signed-off-by: David Marchand <david.marchand@redhat.com> >> > --- >> >> Acked-by: Aaron Conole <aconole@redhat.com> >> >> Somethings that concern me: >> >> 1. A log2(0) should probably be an undetermined value, but this >> effectively makes log2(0) == log2(1) so that if anyone uses these >> for some mathematical work, it will have an exceptional behavior. I >> know it's documented from a programmer perspective, but I am all for >> documenting the mathematical side effect as well. > > What do you have in mind, adding a big warning in the doxygen "THIS IS > NOT REAL MATH STUFF" ? :-) Is such a warning not reasonable? :-) >> >> 2. Why hasn't this been complaining for so long? Or has it and we just >> haven't noticed? Were some compiler flags changed recently? (maybe >> -funsafe-math was always set or something?) > > Yes, I wondered too how we did not see this earlier. > Even now, it happens randomly. > > libc log2(0) is not undefined itself, it returns -infinity. > Looking at the test code, ceilf (I would expect ceil) returns > -infinity when getting it passed. > So I'd say the problem lies in the cast to uint32_t. > > Both gcc and clang do not seem to bother with standard compilation > flags, and the cast gives 0 on my RHEL. > > #include <stdio.h> > #include <inttypes.h> > #include <math.h> > > int main(int argc, char *argv[]) > { > printf("%lf %f %"PRIu32"\n", log2(0), (float)log2(0), (uint32_t)log2(0)); > return 0; > } > > $ ./log2 > -inf -inf 0 > > > Now, if I use UBSAN with clang, I get a complaint at runtime: > $ ./log2 > (/home/dmarchan/log2+0x41dfa5): runtime error: value -inf is outside > the range of representable values of type 'unsigned int' > -inf -inf 0 > > Not sure if it explains the random failures, but won't undefined > behavior eat your babies? Possibly. I would still expect it to be consistent when it eats babies, but maybe it doesn't have to be.

We recently started to get random failures on the common_autotest ut with clang on Ubuntu 16.04.6. Example: https://travis-ci.com/DPDK/dpdk/jobs/263177424 Wrong rte_log2_u64(0) val 0, expected ffffffff Test Failed The ut passes 0 to log2() to get an expected value. Quoting log2 / log(3) manual: If x is zero, then a pole error occurs, and the functions return -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively. rte_log2_uXX helpers handle 0 as a special value and return 0. Let's have dedicated tests for this case. Fixes: 05c4345ef5c2 ("test: add unit test for integer log2 function") Cc: stable@dpdk.org Signed-off-by: David Marchand <david.marchand@redhat.com> Acked-by: Aaron Conole <aconole@redhat.com> --- Changelog since v1: - added comment in rte_log2_uXX API descriptions, --- app/test/test_common.c | 14 +++++++++++++- lib/librte_eal/common/include/rte_common.h | 6 ++++++ 2 files changed, 19 insertions(+), 1 deletion(-) diff --git a/app/test/test_common.c b/app/test/test_common.c index 2b856f8ba5..12bd1cad90 100644 --- a/app/test/test_common.c +++ b/app/test/test_common.c @@ -216,7 +216,19 @@ test_log2(void) const uint32_t max = 0x10000; const uint32_t step = 1; - for (i = 0; i < max; i = i + step) { + compare = rte_log2_u32(0); + if (compare != 0) { + printf("Wrong rte_log2_u32(0) val %x, expected 0\n", compare); + return TEST_FAILED; + } + + compare = rte_log2_u64(0); + if (compare != 0) { + printf("Wrong rte_log2_u64(0) val %x, expected 0\n", compare); + return TEST_FAILED; + } + + for (i = 1; i < max; i = i + step) { uint64_t i64; /* extend range for 64-bit */ diff --git a/lib/librte_eal/common/include/rte_common.h b/lib/librte_eal/common/include/rte_common.h index 459d082d14..7a98071ffe 100644 --- a/lib/librte_eal/common/include/rte_common.h +++ b/lib/librte_eal/common/include/rte_common.h @@ -538,6 +538,9 @@ rte_bsf32_safe(uint64_t v, uint32_t *pos) /** * Return the rounded-up log2 of a integer. * + * @note Contrary to the logarithm mathematical operation, + * rte_log2_u32(0) == 0 and not -inf. + * * @param v * The input parameter. * @return @@ -632,6 +635,9 @@ rte_fls_u64(uint64_t x) /** * Return the rounded-up log2 of a 64-bit integer. * + * @note Contrary to the logarithm mathematical operation, + * rte_log2_u32(0) == 0 and not -inf. + * * @param v * The input parameter. * @return -- 2.23.0

```
On Fri, Dec 20, 2019 at 3:02 PM David Marchand
<david.marchand@redhat.com> wrote:
> diff --git a/lib/librte_eal/common/include/rte_common.h b/lib/librte_eal/common/include/rte_common.h
> index 459d082d14..7a98071ffe 100644
> --- a/lib/librte_eal/common/include/rte_common.h
> +++ b/lib/librte_eal/common/include/rte_common.h
> @@ -538,6 +538,9 @@ rte_bsf32_safe(uint64_t v, uint32_t *pos)
> /**
> * Return the rounded-up log2 of a integer.
> *
> + * @note Contrary to the logarithm mathematical operation,
> + * rte_log2_u32(0) == 0 and not -inf.
> + *
> * @param v
> * The input parameter.
> * @return
> @@ -632,6 +635,9 @@ rte_fls_u64(uint64_t x)
> /**
> * Return the rounded-up log2 of a 64-bit integer.
> *
> + * @note Contrary to the logarithm mathematical operation,
> + * rte_log2_u32(0) == 0 and not -inf.
_u64*
Will fix while applying.
--
David Marchand
```

```
On Fri, Dec 20, 2019 at 3:02 PM David Marchand
<david.marchand@redhat.com> wrote:
>
> We recently started to get random failures on the common_autotest ut with
> clang on Ubuntu 16.04.6.
>
> Example: https://travis-ci.com/DPDK/dpdk/jobs/263177424
>
> Wrong rte_log2_u64(0) val 0, expected ffffffff
> Test Failed
>
> The ut passes 0 to log2() to get an expected value.
>
> Quoting log2 / log(3) manual:
> If x is zero, then a pole error occurs, and the functions return
> -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.
>
> rte_log2_uXX helpers handle 0 as a special value and return 0.
> Let's have dedicated tests for this case.
>
> Fixes: 05c4345ef5c2 ("test: add unit test for integer log2 function")
> Cc: stable@dpdk.org
>
> Signed-off-by: David Marchand <david.marchand@redhat.com>
> Acked-by: Aaron Conole <aconole@redhat.com>
Applied with mentioned fix.
--
David Marchand
```

```
David Marchand <david.marchand@redhat.com> writes:
> On Fri, Dec 20, 2019 at 3:02 PM David Marchand
> <david.marchand@redhat.com> wrote:
>>
>> We recently started to get random failures on the common_autotest ut with
>> clang on Ubuntu 16.04.6.
>>
>> Example: https://travis-ci.com/DPDK/dpdk/jobs/263177424
>>
>> Wrong rte_log2_u64(0) val 0, expected ffffffff
>> Test Failed
>>
>> The ut passes 0 to log2() to get an expected value.
>>
>> Quoting log2 / log(3) manual:
>> If x is zero, then a pole error occurs, and the functions return
>> -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.
>>
>> rte_log2_uXX helpers handle 0 as a special value and return 0.
>> Let's have dedicated tests for this case.
>>
>> Fixes: 05c4345ef5c2 ("test: add unit test for integer log2 function")
>> Cc: stable@dpdk.org
>>
>> Signed-off-by: David Marchand <david.marchand@redhat.com>
>> Acked-by: Aaron Conole <aconole@redhat.com>
>
> Applied with mentioned fix.
Thanks!
```