[PATCH 1/1] Keep the denormal-operand exception masked; modify FE_ALL_EXCEPT accordingly.

Houder houder@xs4all.nl
Wed Aug 15 15:01:00 GMT 2018

On 2018-08-15 16:54, Corinna Vinschen wrote:
> On Aug 15 12:59, J.H. van de Water wrote:
>> By excluding the denormal-operand exception from FE_ALL_EXCEPT, it 
>> will not
>> be possible anymore to UNmask this exception by means of the API 
>> defined by
>> /usr/include/fenv.h
>> Note: terminology has changed since IEEE Std 854-1987; denormalized 
>> numbers
>> are called subnormal numbers nowadays.
>> This modification has basically been motivated by the fact that it is 
>> also
>> not possible on Linux to manipulate the denormal-operand exception by 
>> means
>> of the interface as defined by /usr/include/fenv.h. This has been the 
>> state
>> of affairs on Linux since 2001 (Andreas Jaeger).
>> The exceptions required by the standard (IEEE Std 754), in case they 
>> can be
>> supported by the implementation, are:
>> Although it is allowed to define additional exceptions, there is no 
>> reason
>> to support the "denormal-operand exception" in this case (fenv.h), 
>> because
>> the subnormal numbers can be handled almost as fast the normalized 
>> numbers
>> by the hardware of the x86/x86_64 architecture. Said differently, a 
>> reason
>> to trap on the input of subnormal numbers does not exist. At least 
>> that is
>> what William Kahan and others at Intel asserted around 2000.
>> (that is William Kahan of the K-C-S draft, the precursor to the 
>> standard)
>> This commit modifies winsup/cygwin/include/fenv.h as follows:
>>  - redefines FE_ALL_EXCEPT from 0x3f to 0x3d
>>  - removes the definition for FE_DENORMAL
>>  - introduces __FE_DENORM (0x2) (enum in Linux also uses __FE_DENORM)
>>  - introduces FE_ALL_EXCEPT_X86 (0x3f), i.e. ALL x86/x86_64 FP 
>> exceptions
> Shouldn't FE_ALL_EXCEPT_X86 be defined locally in fenv.cc only?
> I don't see that Linux exports that definition.

Ah, Sorry. Do I have to resubmit my patch? Or is it easy enough for you 
make this modification?


More information about the Cygwin-patches mailing list