test/common: fix log2 check
diff mbox series

Message ID 20191204205241.5691-1-david.marchand@redhat.com
State Superseded, archived
Delegated to: David Marchand
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Series
  • test/common: fix log2 check
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Commit Message

David Marchand Dec. 4, 2019, 8:52 p.m. UTC
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(-)

Comments

Aaron Conole Dec. 4, 2019, 9:19 p.m. UTC | #1
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
David Marchand Dec. 5, 2019, 8:31 a.m. UTC | #2
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?
Aaron Conole Dec. 5, 2019, 2:39 p.m. UTC | #3
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.

Patch
diff mbox series

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 */