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Re: [Patch, Fortran] Enable flag_associative_math (almost) by default
ping**3 -- and last call. If I do not get any feedback, I will commit
it tomorrow as obvious :P
Tobias
On 05/17/2010 10:41 AM, Tobias Burnus wrote:
> ping**2
>
> On 05/10/2010 01:37 PM, Tobias Burnus wrote:
>
>> *ping*
>> http://gcc.gnu.org/ml/fortran/2010-05/msg00033.html
>>
>> On 05/06/2010 02:24 PM, Tobias Burnus wrote:
>>
>>
>>> The GCC manual claims:
>>>
>>> -fassociative-math
>>> Allow re-association of operands in series of floating-point operations.
>>> This violates the ISO C and C++ language standard by possibly changing
>>> computation result. [...] This option requires that both
>>> -fno-signed-zeros and -fno-trapping-math be in effect. [...] For Fortran
>>> the option is automatically enabled when both -fno-signed-zeros and
>>> -fno-trapping-math are in effect.
>>>
>>> And related the gfortran manual has:
>>>
>>> -fno-protect-parens
>>> By default the parentheses in expression are honoured for all
>>> optimization levels such that the compiler does not do any
>>> re-association. [...] Note that for the reassociation optimization
>>> -fno-signed-zeros and -fno-trapping-math need to be in effect.
>>>
>>> If one now looks for flag_associative_math one sees that gfortran does
>>> not touch it at all and also toplev.c has only the trapping/signed zero
>>> check and no special case for Fortran.
>>>
>>> Well, the following patch does what is written in the manual; as
>>> -funsave-math-optimization implies both
>>> -fno-trapping-math/-fno-signed-zeros and -fassociate-math -- and as
>>> -ffast-math implies -funsave-math-optimizations, the practical impact of
>>> the change is small.
>>>
>>> Note: Even with -ffast-math -fassociate-math, the parentheses are
>>> protected (unless -fno-protect-parens is used).
>>>
>>> Build on x86-64-linux.
>>> OK for the trunk?
>>>
>>> Tobias
>>>
>>> PS: Thanks go to Dominique pointed out the error in the documentation.
>>>
>>> PPS: There is also a "(!flag_trapping_math && !flag_signed_zeros)" check
>>> in toplev.c, which prevents enabling -freassociative-math - but the
>>> function prints an ignore warning. Otherwise, one could simply set the
>>> flag to 1 in the option init section.
>>>
>>>
>>>
>>
>>
>
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