What I'm talking about ist the following:
CRT_fp10.o on startup executes FNINIT. According to
http://www.intel.com/Assets/ja_JP/PDF/manual/253665.pdf:
"When the x87 FPU is initialized with either an FINIT/FNINIT or
FSAVE/FNSAVE instruction, the x87 FPU control word is set to 037FH, which
masks all floating-point exceptions, sets rounding to nearest, and sets the
x87 FPU precision to 64 bits."
That is 80-bit double extended computation precision as default and
definitly not the expected double computation precision. See also:
http://sourceforge.net/p/mingw/bugs/904/
https://ghc.haskell.org/trac/ghc/ticket/7289
Carl
2014-11-23 10:52 GMT+01:00 Yaron Keren <yaron.ke...@gmail.com>:
> http://msdn.microsoft.com/en-us/library/e9b52ceh.aspx
>
> I think three issues are conflated:
>
> 1 long double size is 80 bits in mingw and 64 bits in VC.
>
> 2 The mask 0x27F will set the 80-bit extended computation precision,
> _PC_64 (64/80) and not _PC_53 (53/64) computation precision. This is
> internal FPU computation precision regardless the external representation
> size.
>
> 3 The exceptions Carl refers are disabled by setting the _MCW_EM mask,
> mostly 0x1F of the 0x27F mentioned. This is independent the computation
> precision or long double size.
>
> see also
> http://www.exploringbinary.com/when-doubles-dont-behave-like-doubles/
>
>
>
> 2014-11-23 11:08 GMT+02:00 Kai Tietz <ktiet...@googlemail.com>:
>
>> 2014-11-23 9:17 GMT+01:00 Ozkan Sezer <seze...@gmail.com>:
>> > On 11/12/14, Carl Kleffner <cmkleff...@gmail.com> wrote:
>> >> Hi,
>> >>
>> >> during tests with my mingw builds variant mingw-w64-for-python
>> >> <https://bitbucket.org/carlkl/mingw-w64-for-python/downloads> I
>> stumpled
>> >> upon a flaw in mingw-w64 FPU handling on startup. This is issued at
>> >> mingw-w64 builds of numpy <https://github.com/numpy/numpy/issues/5194>
>> and
>> >> test
>> >> failures when building win32 binaries with mingw based on gcc 4.8
>> >> <https://github.com/numpy/numpy/issues/4909>. The reason for this
>> >> behaviour: CRT_fp10.o (part of libmingw32.a) sets the FPU control word
>> to
>> >> extended precision on startup as per default. This can be overwritten
>> by
>> >> additionally linking CRT_fp8.o, but this is documented elsewhere
>> >> (floath.h). As far I can see, this behaviour in mingw-w64 inherits
>> from the
>> >> Mingw32 codebase, but I'm not sure.
>> >>
>> >> There seems to be a consense, that 027f should be the standard for
>> setting
>> >> the FPU control word. Otherwise it gets into your way, see i.e.
>> Random and
>> >> unexpected EXCEPTION_FLT_DIVIDE_BY_ZERO and
>> >> EXCEPTION_FLT_INVALID__OPERATION
>> >> <
>> http://blogs.msdn.com/b/dougste/archive/2008/11/12/random-and-unexpected-exception-flt-divide-by-zero-and-exception-flt-invalid-operation.aspx
>> >
>> >>
>> >> Is there some code in mingw.w64 that depends on extended precision as a
>> >> default value? Otherwise this should be fixed in the mingw-w64 codebase
>> >> IMHO.
>> >>
>> >> Regards
>> >>
>> >> Carl
>> >
>> > PING ?
>> >
>> > --
>> > O.S.
>>
>> Well, there is AFAIK. We have the 'long double' support, which is
>> actually using 80-bit extended-mode x87 FPU,
>>
>> So, not sure if need to keep it for this.
>>
>> Kai
>>
>>
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