On 16/05/11 11:45, Pawel Sikora wrote: > On Monday 16 of May 2011 11:15:29 Andrew Haley wrote: >> 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. > > Hi, > > i've naturally reported these numerical problems to the author at first place > but i'm still impressed that code produced by gcc for x87/x86-64 with explicit > and equal -mpc32/-mfpmath options gives different results. > > should -mpc32 and equal fpmath model produce equal results (no matter good or bad) ? Not necessarily. Whatever libraries your code is calling won't be affected by the compiler options you use, for example. > or mabye there's a bug in gcc exposed by explicit -fexcess-precision option? Maybe. > shoud i report this as potential gcc bug? No, because we haven't even established that there is a gcc bug yet. There's little point in reporting a bug without a test case that shows what gcc is doing wrong. In general, floating-point on 64-bit x86 is better behaved than on 32-bit. Andrew.
