Boost C++ Libraries: Ticket #10924: conversion of float/double to cpp_dec_float_100 causes hang before main() on cygwin

cc set

John Maddock — Sun, 18 Jan 2015 19:04:09 GMT

cc e_float@… added

Works for me with Mingw64 and msvc. Can you debug? What happens in GDB - does it get stuck in an infinite loop somewhere?

anonymous — Sun, 18 Jan 2015 19:59:50 GMT

The most computationally intensive operation in cpp_dec_float before main() is the generation of a static constant table of p^{2 for -128 < p < +128.}

It would help us if you could std::cout << "something clever" before and after the table is generated ?

If you look near line 2345 in boost/cpp_dec_float.hpp, you will find a potential location for this diagnostic.

Thank you for your report.

status changed; resolution set

John Maddock — Fri, 06 Feb 2015 18:38:01 GMT

status new → closed
resolution → fixed

Turns out this is quite a pervasive issue - if there are no long double math functions (as on cygwin) then a call such as:

stdlibfunc(BIG_VALUE)

may simply return infinity as BIG_VALUE gets silently converted to double inside "stdlibfunc". In this particular case it's frexp/ldexp that are basically returning nonsense and breaking the program invariants.

Simplistic fix in https://github.com/boostorg/multiprecision/commit/4340817fd818b80df2295060eafdf79549d88683

John Maddock — Fri, 06 Feb 2015 19:03:38 GMT

Also added some assertions for the preconditions which get broken on cygwin: https://github.com/boostorg/multiprecision/commit/ca1f4697724fbb3dc3afed465e2e027ef4d4522d