question about equivalent x87/x64-64 fpu code...

[Andrew Haley]

On 13/05/11 19:11, Paweł 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.