Re: Incorrect results from numerical library when using -ftree-ter
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On 8 June 2012 00:50, Ian Lance Taylor <iant@xxxxxxxxxx> wrote: > Szabolcs Horvát <szhorvat@xxxxxxxxx> writes: > >> I am using the Triangle library >> <http://www.cs.cmu.edu/~quake/triangle.html> to generate Delaunay >> triangulations of a large number of points. When I compile this >> library using the -ftree-ter optimization option only, the library >> gives me incorrect results for certain inputs. The -ftree-ter option >> is enabled by optimization level -O1 or higher, which also cause the >> library to return incorrect results. >> >> Question: Does this indicate that there is a bug in gcc or is it more >> likely that there's a problem in how the library is written? >> >> Unfortunately I am not familiar with the code of Triangle, I just >> compile it and use it. >> >> I tried this with gcc 4.6.3 on Ubuntu and gcc 4.7.0 on Windows (from >> <http://nuwen.net/mingw.html>), both 32-bit. > > > While a bug in GCC is always possible, it is more likely that there's a > problem in how the library is written. > Unfortunately I am not in a position to find out if it's a problem with Triangle as I don't know in detail how it works. Triangle is quite extensively used though, and it's last version is from 2005. I wonder why the problem has not come out before. A strange thing is that I get a different kind of incorrect result if I use the -O1 option or if I manually specify all the flags that -O1 is supposed to turn on. What could be the reason for this? I got the list of flags from here: http://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Optimize-Options.html#Optimize-Options Similarly, using -O1 -fno-tree-ter does NOT fix the problem. Specifying all options that -O1 is supposed to imply according to the docs, except -ftree-ter DOES fix the problem. What is the difference between -O1 and listing all options that it implies?