On 13/05/11 19:11, PaweÅ Sikora wrote: > Hi, > > i'm using a 3rd-party engine http://glaros.dtc.umn.edu/gkhome/metis/metis/overview > for partitioning some complex data. it worked fine for years until today (may 13)... > > observations: > - the 32-bit metis build produces nice and balanced partitons. > - the 64-bit metis build produces bad and unbalanced partitons. > > the metis' engine uses arrays of integers on the public interface and internally > some float-based and unsafe in terms of precison (x<y and x==y) operations. > > so, i've built/tested following metis variants: > > 1). -m32 -march=pentium4 -O1 - works fine. > 2). -m32 -march=pentium4 -O1 -mfpmath=sse - works fine. > 3). -m64 -march=x86-64 -O1 - bad/unbalanced partitions. > 4). -m64 -march=x86-64 -O1 -mfpmath=387 - bad/unbalanced partitions. > > at this point i've expected wrong results (< 80-bit precision) from variants 2/3 > and good results from variants 1/4 but the real world differs. > > next, i've isolated a one place in sources with float x<y stmt and changed it > to (x-y)<0.00001. with such change both native 1/3 variants give nice/equivalent results. > > so, where is the problem? is the variants 1/4 really equivalent? It's going to be very hard for gcc specialists to answer this. You really need a numerical analyst who is familiar with the code to have a look. This may be a gcc bug, or it may be a bug in the code. It'd impossible to know without doing more digging into the problem. Andrew.
