Hi Jon! Thanks for your reply.
On Sat, Jul 20, 2013 at 12:17 PM, JonY <jo...@users.sourceforge.net> wrote: > On 7/20/2013 23:43, K. Frank wrote: >> Hello List! >> >> On 64-bit mingw-w64: >> >> g++ (rubenvb-4.8-stdthread) 4.8.1 20130324 (prerelease) >> >> on 64-bit windows 7, I'm seeing that long doubles have a precision of about >> 18 decimal digits. I would guess that this is a legacy of the old 8087 >> 80-bit >> internal floating-point number. >> >>>From a quick test program (no explicit compiler flags): >> ... >> numeric_limits<long double>::digits10 = 18 >> ... >> numeric_limits<long double>::max_digits10 = 21 >> ... >> sizeof(long double) = 16 >> >> As you can see, long double offers only a modest increase in precision over >> double, but takes fully twice the space. >> >> Also, Intel's old 80-bit floating-point format fits in five 16-bit >> words (which fits in >> three 32-bit words). I am guessing that alignment issues on a 64-bit machine >> make it not worth the bother to use less than a full two 64-bit words to >> hold a >> long double, even though (if I'm right about the 80-bit >> representation), there's a >> fair amount of wasted space. >> >> Does this all sound right? > > on a 64-bit machine, floating point goes on SSE registers, which support > 2x64-bit floats (128-bit per xmm reg). On a 32-bit machine, it goes on > the 80-bit x87 fpu. 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? > 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. Thanks for any additional wisdom. K. Frank ------------------------------------------------------------------------------ See everything from the browser to the database with AppDynamics Get end-to-end visibility with application monitoring from AppDynamics Isolate bottlenecks and diagnose root cause in seconds. Start your free trial of AppDynamics Pro today! http://pubads.g.doubleclick.net/gampad/clk?id=48808831&iu=/4140/ostg.clktrk _______________________________________________ Mingw-w64-public mailing list Mingw-w64-public@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/mingw-w64-public
