RE: Problem with source.f

From: Chris Theis (
Date: Sat Nov 04 2006 - 14:43:57 CET

  • Next message: Joseph Comfort: "Re: Problem with source.f"

    Dear Joseph,

    I certainly don't mean to get into a discussion about when/where/why
    double precision is mandatory or about any arguments regarding quality.

    But I rather want to comment on one myth that I'm sure will come up
    again in the near future and is probably of interest to the simulation
    community, whatever code they use. It's about precision and 64 bit
    machines/compilations. I've developed my own CSG kernel and tracking
    code so I had happily jumped onto the idea that 64-bit will help me to
    get rid of numerical accuracy issues as it will give us better floating
    point accuracy. However, when I saw the specs of the 64-bit machines I
    learnt that this is a misconception, which is actually widely spread.
    Practically the FPU of all modern processors complies with the IEEE754
    standard which specifies the precision and also the 64-bit processors
    adhere to this specification. Basically, this means that single
    precision is represented with 32 bits and double precision with 64 bits.
    However, also the 32-bit Intel/AMD processors already complied with this
    standard, so there is no change at all with respect to the floating
    point accuracy in 64-bit compilations. Still, the issue is more subtle
    because in comparing the results of 32 & 64 bit programs one has to be
    very very careful to restrict the compilers really to exactly identical
    settings. The reason is that even with 32-bit machines there is
    something which is called extended double precision, which basically
    allows floating point calculations to take place using 80 bits.
    Nevertheless, the results are truncated to 64 bits before being written
    into memory. Even single precision calculations (32-bits) will be
    promoted to 80 bits if this mode is activated, which is something that
    most people outside the compiler development community are unaware of.
    If this extended double precision mode is used or not depends on the
    compiler and its settings and defaults. Therefore, one has to take
    immense care to specify really the same compilation environment for 32
    and 64 bit comparisons.

    What 64 bit machines will give you is that you can address more memory
    and that some programs _may_ run faster, but this discussion would lead
    too far and would get very technical.

    Best regards


    Chris Theis
    CERN/SC-RP - European Organization for Nuclear Research
    1211 Geneva 23, Switzerland
    Phone: +41 22 767 8069 Office: 892-2A-015

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