Re: [BLACKBOX] Math.Pi (was: CONSTant expressions)

From: Campbell, Robert (SELEX GALILEO, UK) <"Campbell,>
Date: Thu, 7 Feb 2008 09:53:40 -0000

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>> Chris Burrows wrote:
>> >> Additionally, how does your theory explain this one?
>> >>
>> >> OutBool ('Pi = Pi ? ', Math.Pi() = Math.Pi(); (*
FALSE *)


Robert Campbell replied:
>> With the best will in the world, this *must* be a bug, that
>> needs fixing. Urgently.


Hi

I withdraw my comment above. The compiler computes intermediate results
to 80
bits, and leaves them in 80 bit FPU (Floating Point Unit) registers when
it can.
When it can't it copies rounded intermediate results into 64 bit REAL
registers.

The first Math.Pi result is copied off the FPU stack before the second
Math.Pi is called.
Hence the comparison is between a 64 bit value and an 80 bit value.

This strategy provides the least roundoff error in a language that does
not pretend
to support an 80 bit extended precision longreal type, and I do not
consider it a bug.
Indeed I fully support it.



My comment above was partly triggered by a rather careless
misunderstanding of a test
programme I wrote. Thanks to Oms for pointing out the error in my
thinking.


Regards

Robert

SELEX Sensors and Airborne Systems Limited
Registered Office: Sigma House, Christopher Martin Road, Basildon, Essex SS14 3EL
A company registered in England & Wales. Company no. 02426132
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Received on Thu Feb 07 2008 - 10:53:40 UTC