On 2020-11-29 19:14:37 +0300, Alex Markin via Gcc-help wrote: > The problem starts when you have a cross-platform project where on > the second platform we have a non-gcc compiler and 80-bit float > calculations on non-intel hardware. That is my case where on the > second platform expl calculates in the right way and after a > sequence of evaluations we get a big error accumulation and the > program behaviour becomes different. This is not specific to the 80-bit extended precision. Any FP format may be affected. When the argument is known at compile time, GCC can optimize and it normally uses GNU MPFR to compute the result with correct rounding (which is the best possible result and the only good way to specify the result completely). This means that if the library does not provide correct rounding, you will get differences with results computed at compile time (and differences with other libraries, possibly across different versions of the same library). You can see a summary of differences between libraries for the double type (double precision, or binary64) and various functions: https://www.vinc17.net/research/testlibm/ These are tests on values that are difficult to round due to the "Table maker's dilemma". As you can see, for a long time, glibc was implementing correct rounding for exp and log in particular (my tests started with glibc 2.2.5, on spe170 in the table), thus with no differences compared to the compile-time result with GCC (using MPFR); but this is no longer the case (e.g. with glibc 2.28, on joooj in the table). This changed in 2018: commit de800d83059dbedb7d151580f0a3bdc9eaf37340 Author: Szabolcs Nagy <szabolcs.nagy@xxxxxxx> Date: 2018-01-30 15:48:22 +0100 Remove slow paths from exp commit b7c83ca30ef8e85b6642151d95600a36535f8d97 Author: Wilco Dijkstra <wdijkstr@xxxxxxx> Date: 2018-02-07 13:24:43 +0100 Remove slow paths from log (BTW, reducing the precision of the slow path may have been a better compromise, with no differences on the results, at least on exp and log, as I say in the slides mentioned below.) On https://www.vinc17.net/research/slides/sieste2010.pdf slides 11 to 17 you can see various differences obtained with GCC involving a same expression. -- Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <https://www.vinc17.net/> 100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/> Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
