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Re^3: powerpc double-double arithmetic

by bulk88 (Priest)
on Oct 10, 2013 at 03:10 UTC ( #1057667=note: print w/ replies, xml ) Need Help??


in reply to Re^2: powerpc double-double arithmetic
in thread powerpc double-double arithmetic

From what I know about the x86 long double is that, for PPC, I guess sizeof is returning 16 for alignment reasons. 12 is the actual used portion. A long double is 80 bits/10 bytes on x86. Mingw claims it is 12 bytes in a sizeof.


Comment on Re^3: powerpc double-double arithmetic
Re^4: powerpc double-double arithmetic
by syphilis (Canon) on Oct 10, 2013 at 03:44 UTC
    for PPC, I guess sizeof is returning 16 for alignment reasons

    With PPC, it actually uses the 16 bytes (in the form of 2 8-byte doubles).
    There are other processors that use 12 bytes but assign 16 bytes (for alignment purposes, as you said).

    Cheers,
    Rob
      So if its vector math op (SSE, etc), its really 2 64 bit doubles and not a "long double". Long double would mean over 64 bits of double/precision. You have "packed data" not a "long double". Does your CPU have 128 bit FP math (see also) or not? Does your compiler emulate 128 bit FP math (GCC yes as __float128)?
        The basics of its arithmetic is here.
        There's no 128 bit FP math with its gcc-4.6.3 compiler. I think the double-double technique precludes it. (Every other gcc-4.6.3 that I've seen has a quadmath.h that provides the __float128 type, but not this one - which is provided by Debian specifically for the PowerPC architecture.)

        Cheers,
        Rob

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