Re: [PATCH v2 1/7] powerpc: Do not raise exception traps for fesetexcept/fesetexceptflag (BZ 30988)

[Carlos O'Donell <carlos@redhat.com>]

On 11/6/23 12:11, Adhemerval Zanella Netto wrote:
> 
> 
> On 06/11/23 14:02, Carlos O'Donell wrote:
>> On 11/6/23 11:50, Adhemerval Zanella Netto wrote:
>>>
>>>
>>> On 06/11/23 13:08, Carlos O'Donell wrote:
>>>> On 11/6/23 08:27, Adhemerval Zanella wrote:
>>>>> According to ISO C23 (7.6.4.4), fesetexcept is supposed to set
>>>>> floating-point exception flags without raising a trap (unlike
>>>>> feraiseexcept, which is supposed to raise a trap if feenableexcept was
>>>>> called with the appropriate argument).
>>>>>
>>>>> This is a side-effect of how we implement the GNU extension
>>>>> feenableexcept, where feenableexcept/fesetenv/fesetmode/feupdateenv
>>>>> might issue prctl (PR_SET_FPEXC, PR_FP_EXC_PRECISE) depending of the
>>>>> argument.  And on PR_FP_EXC_PRECISE, setting a floating-point exception
>>>>> flag triggers a trap.
>>>>>
>>>>> To make the both functions follow the C23, fesetexcept and
>>>>> fesetexceptflag now fail if the argument may trigger a trap.
>>>>
>>>> OK. I reviewed ISO C 2x (n3096), and I agree this is permissible and preferable.
>>>>
>>>>>
>>>>> The math tests now check for an value different than 0, instead
>>>>> of bail out as unsupported for EXCEPTION_SET_FORCES_TRAP.
>>>>>
>>>>> Checked on powerpc64le-linux-gnu.
>>>>
>>>> Changes test from UNSUPPORTED to PASS when we should test more now that with
>>>> C2x we're saying the behaviour will result in a non-zero return... then we
>>>> should test for that.
>>>>
>>>>> ---
>>>>>  math/test-fesetexcept-traps.c      | 11 ++++-------
>>>>>  math/test-fexcept-traps.c          | 11 ++++-------
>>>>>  sysdeps/powerpc/fpu/fesetexcept.c  |  5 +++++
>>>>>  sysdeps/powerpc/fpu/fsetexcptflg.c |  9 ++++++++-
>>>>>  4 files changed, 21 insertions(+), 15 deletions(-)
>>>>>
>>>>> diff --git a/math/test-fesetexcept-traps.c b/math/test-fesetexcept-traps.c
>>>>> index 71b6e45b33..96f6c4752f 100644
>>>>> --- a/math/test-fesetexcept-traps.c
>>>>> +++ b/math/test-fesetexcept-traps.c
>>>>> @@ -39,16 +39,13 @@ do_test (void)
>>>>>        return result;
>>>>>      }
>>>>>  
>>>>> -  if (EXCEPTION_SET_FORCES_TRAP)
>>>>> -    {
>>>>> -      puts ("setting exceptions traps, cannot test on this architecture");
>>>>> -      return 77;
>>>>> -    }
>>>>> -  /* Verify fesetexcept does not cause exception traps.  */
>>>>> +  /* Verify fesetexcept does not cause exception traps.  For architectures
>>>>> +     where setting the exception might result in traps the function should
>>>>> +     return a nonzero value.  */
>>>>>    ret = fesetexcept (FE_ALL_EXCEPT);
>>>>>    if (ret == 0)
>>>>
>>>> We can check for a non-zero return if EXCEPTION_SET_FORCES_TRAP?
>>>>
>>>> e.g.
>>>>
>>>>   if (!EXCEPTION_SET_FORCES_TRAP)
>>>>     { 
>>>>       if (ret == 0)
>>>>         puts ("fesetexcept (FE_ALL_EXCEPT) succeeded");
>>>>       else
>>>>         /* fail */
>>>>     }
>>>>   else
>>>>     {
>>>>       if (ret == 0)
>>>>         /* fail */
>>>>       else
>>>>         /* pass */
>>>>     }
>>>
>>> The '!EXCEPTION_SET_FORCES_TRAP && ret == 0' or 'EXCEPTION_SET_FORCES_TRAP && ret == 1'
>>> checks are not really meaningful: either the function succeeds and return 0, or it fails
>>> for some reason.  And for failure, EXCEPTION_SET_FORCES_TRAP really means an expected 
>>> failure.
>>
>> Sure.
>>
>>> So if the function succeeds and no trap is generated (which terminates the process
>>> as default on Linux) we are fine.  Otherwise, it check if the failure is expected
>>> (EXCEPTION_SET_FORCES_TRAP).
>>>
>>
>> So we go from UNSUPPORTED to... ?
>>
> 
> I though about that, but the test also checks fegetexceptflag (a better option would
> to split the test in two, so only the fesetexceptflag is unsupported on ppc32).
> 

Perhaps the best option is to just keep the UNSUPPORTED status?

-- 
Cheers,
Carlos.