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README.md 3.27 KiB

Requirements

  • g++ v5.4 or newer (compilation was performed with g++ 9.3)
  • Boost 1.58.0 or newer
    • The boost/multiprecision/float128.hpp header must be available
  • Make

Installed Boost Packages

For development the following boost packages were installed.

libboost-date-time1.58.0/xenial-updates,now 1.58.0+dfsg-5ubuntu3.1 amd64 [installed]
libboost-filesystem1.58.0/xenial-updates,now 1.58.0+dfsg-5ubuntu3.1 amd64 [installed]
libboost-iostreams1.58.0/xenial-updates,now 1.58.0+dfsg-5ubuntu3.1 amd64 [installed]
libboost-python1.58.0/xenial-updates,now 1.58.0+dfsg-5ubuntu3.1 amd64 [installed,automatic]
libboost-regex1.58.0/xenial-updates,now 1.58.0+dfsg-5ubuntu3.1 amd64 [installed,automatic]
libboost-system1.58.0/xenial-updates,now 1.58.0+dfsg-5ubuntu3.1 amd64 [installed]
libboost-test-dev/xenial,now 1.58.0.1ubuntu1 amd64 [installed]
libboost-test1.58-dev/xenial-updates,now 1.58.0+dfsg-5ubuntu3.1 amd64 [installed,automatic]
libboost-test1.58.0/xenial-updates,now 1.58.0+dfsg-5ubuntu3.1 amd64 [installed,automatic]
libboost1.58-dev/xenial-updates,now 1.58.0+dfsg-5ubuntu3.1 amd64 [installed,automatic]
libboost1.58-doc/xenial-updates,xenial-updates,now 1.58.0+dfsg-5ubuntu3.1 all [installed]

This listing was obtained by running apt list --installed | grep boost.

Compilation

The code can be compiled with the provided makefile using the standard make command.

If compiling the code manually, or integrating into a larger program, include the following flags:

FLAGS=-std=c++17 -fsanitize=address -fuse-ld=gold -Wall -MMD \
      -fext-numeric-literals -lquadmath #-O3

Note that flag -fuse-ld=gold is only required on certain Ubuntu systems due to a know bug with g++ 5.x.

Sample Execution & Output

If run without command line arguments, using

./precisionEstimate

the following usage message will be displayed.

Usage: ./precisionEstimate numExecs

If run using

./precisionEstimate 100000000

output simliar to

   0 secs | 1.19209e-07
   1 secs | 2.22045e-16
  17 secs | 1.92593e-34

will be displayed. Note that the precision estimates will vary by architecture/system.

Merge Requests

This section documents contributions made by students via merge requests.

Replace naïve float128 abs implementation with something closer to Rust implementation

Author: Alex Launi Date: 2020-05-21

Goal

Remove bottleneck caused by conditional negation implementation of float128 abs.

Results

Note: these were run on a 2019 MacBook Pro using a 2.4 GHz 8-Core Intel Core i9.

Version Time (1x10^9)
Orig (-O0) 326s
Mine (-O0) 179s
Orig (-O3) 92s
Mine (-O3) 60s

Possible Improvement

Rust uses MPFR on the backend. I dug into the MPFR source, and this is close to the implementation used. The primary difference is the use of a MPFR_SET_SIGN macro. Boost has a copysign(const T& x, const T& y) which copies the sign from y to x. Something like

inline
float128 abs(float 128 x)
{
    const float128 p = 1.0Q;
    return copysign(x, p);
}

should work, but I was unable to get any use of copysign to compile. I settled on the approach used in this merge request. I think the results speak well.