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 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... ?

-- 
Cheers,
Carlos.