On 7/21/2013 01:58, K. Frank wrote: > > That would certainly make sense, but it doesn't square with what > numeric_limits is telling me: > > numeric_limits<long double>::digits10 = 18 > numeric_limits<long double>::max_digits10 = 21 > > Just to check that numeric_limits isn't lying to me I ran a small program > that adds and subtracts 1.0 to and from a small long double. This agrees > with numeric_limits and shows about 20 decimal digits of precision. > > --------------- > d = 1e-017 > (d += 1.0; d -= 1.0;) > d = 9.97466e-018 > --------------- > d = 1e-018 > (d += 1.0; d -= 1.0;) > d = 9.75782e-019 > --------------- > d = 1e-019 > (d += 1.0; d -= 1.0;) > d = 1.0842e-019 > --------------- > d = 1e-020 > (d += 1.0; d -= 1.0;) > d = 0 > --------------- > > I compiled the test program without any special compiler flags. Do I > need to somehow enable the SSE registers you mentioned in order > to take advantage of them? >
I checked the assembly listing and have confirmed that GCC still tries to use the 387 for extended precision and will use SSE for double precision. IIRC AMD's 387 implementation is 64bit only, so your results might differ on them. >> Use something like MPFR if you want arbitrary high precision math >> independent of hardware capabilities. > > No, I don't need that. I was just a little curious about what I saw, and > thought an 80-bit long double was a little skimpy for a modern, 64-bit > machine. There is quad precision floats with 128-bit lengths, they're not very commonly supported. GCC supports it, but aren't any math routines in mingw-w64 to actually work on it, eg quad sqrt etc. There is also decimal math support in GCC, there is some work on the math routines, but nothing yet for general use.
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