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Complex*32

 
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MarkJordan



Joined: 18 Jan 2006
Posts: 11

PostPosted: Mon Apr 20, 2020 3:48 pm    Post subject: Complex*32 Reply with quote

I know I can get to complex*20 - 18 digit precision.
Is there any support for higher precision?
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PaulLaidler
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Joined: 21 Feb 2005
Posts: 6563
Location: Salford, UK

PostPosted: Mon Apr 20, 2020 4:51 pm    Post subject: Reply with quote

complex*20 is available with FTN95 but only for Win32. Higher precision is not supported.
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John-Silver



Joined: 30 Jul 2013
Posts: 1450
Location: Aerospace Valley

PostPosted: Mon Apr 20, 2020 8:43 pm    Post subject: Reply with quote

is there a technical reason why it's not available in x64 ?
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PaulLaidler
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Joined: 21 Feb 2005
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PostPosted: Mon Apr 20, 2020 9:36 pm    Post subject: Reply with quote

The hardware does not provide it.
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JohnCampbell



Joined: 16 Feb 2006
Posts: 2227
Location: Sydney

PostPosted: Tue Apr 21, 2020 2:51 am    Post subject: Reply with quote

Paul,

Is there a more extensive explanation ?
Is a 8087 - 80 bit hardware supported in Win32 or is it emulated in 32-bit ?
If it is available from Win32, why can't it be accessed from x64 ?

I did have a use for 80 bit use in eigenvector "filtering" (Gram-Schmidt Orthogonalization), where I was calculating a tri-product of 2 eigenvectors and mass vector. I thought I proved that 80-bit filtering was better than 64-bit, although I gave that away with the 64-bit conversion. One of those projects on the to-do list. Might get a Covid bump up the list !

John
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PaulLaidler
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Joined: 21 Feb 2005
Posts: 6563
Location: Salford, UK

PostPosted: Tue Apr 21, 2020 7:57 am    Post subject: Reply with quote

John

There will be but I don't have the experience and knowledge to give it.

With my limited understanding, if I wanted 80 bit reals in a 64 bit executable then I would set up a pipe to a 32 bit executable running in the background that would access the FPU. As I understand it, if there is a direct way to get to the FPU then Microsoft do not support it.
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Robert



Joined: 29 Nov 2006
Posts: 346
Location: Manchester

PostPosted: Tue Apr 21, 2020 6:31 pm    Post subject: Reply with quote

I think the hardware does provide it but it is difficult to use in x64. 80-bit reals use the FP stack model whereas efficient x64 code uses SSE instructions. 80-bit reals are not supported in .NET either
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JohnCampbell



Joined: 16 Feb 2006
Posts: 2227
Location: Sydney

PostPosted: Wed Apr 22, 2020 4:16 am    Post subject: Reply with quote

It has been a puzzle to me to know if there is still an "80-bit 8087" like FPU on modern processors. Certainly we are encouraged to use SSE or AVX instructions due to their efficiency. But if an 80-bit hardware computation existed for, say dot_product_80, it can in theory improve the accuracy, without resorting to a much slower higher precision emulation library.
It is only recently that FTN95 has provided access to SSE and AVX, so haven't we been mostly using 64-bit calculations, without the extra bits that 80-bit registers provide, at no performance cost?

I do have a dot_product_kahan (...), although only used it in test examples.

I never had a FE modelling job where 80 bit made a real difference. Not sure why, as in theory it could, although all the arrays were real*8. I did these tests many years ago.
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Robert



Joined: 29 Nov 2006
Posts: 346
Location: Manchester

PostPosted: Wed Apr 22, 2020 10:14 am    Post subject: Reply with quote

FTN95 x64 has always used the SSE instructions for floating point arithmetic.
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JohnCampbell



Joined: 16 Feb 2006
Posts: 2227
Location: Sydney

PostPosted: Tue May 19, 2020 10:00 am    Post subject: Re: Reply with quote

Robert wrote:
FTN95 x64 has always used the SSE instructions for floating point arithmetic.

Robert, when you say "SSE instructions", do you mean 64-bit (as opposed to 80-bit) ?
I understand FTN95 /OPT does not incorporate any SSE or AVX register/vector instructions for FP calculations.
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