Hi Jan, On Mon, Jun 13, 2011 at 03:54:51PM -0400, Jan Chludzinski wrote: > First, I tried "-march=pentium3 -mfpmath=sse" to no avail - same wrong > answer. The is a "rigid plate" analysis for a vibration problem. The > correct answer is the plate lattice damps out at -0.414329 @ ~0.5 > seconds. With G++ 4.3.4 I'm now getting: -0.43281 @ ~0.9 seconds. > > Second, I'm simply compiling the source code using g++ <source-file>, > the only option "-g". > > ---Jan > > On Mon, Jun 13, 2011 at 6:54 AM, Jonathan Wakely <jwakely.gcc@xxxxxxxxx> wrote: > > On 13 June 2011 11:24, Jan Chludzinski wrote: > >> Just finished compiling some numerical code (developed using the > >> Borland C++ compiler) using G++ 4.3.4 (that came with Cygwin 1.7). The > >> answers are different from what I get using the Borland compiler > >> (circa 2002). I have known correct answers from some NASA code and > >> compare against those. > >> > >> I've transitioned of late to Code::Blocks using the latest MinGW. > >> MinGW comes with G++ 4.5.2. I compiled using this compiler and it > >> once again works (I get the same answers as the NASA code). > >> > >> Are there known problems with G++ 4.3.4? Well, I don't know your code / library but floating point calculations DO depend on compiler, architecture, compilation flags, optimization,... (and the moon phase ;-)). And if a library is not written extremely clean, it's quite easy to introduce numerical instabilities as soon as the numerics become a bit more complex. (Like Jonathan suggested: On x86, it's a difference whether numerics are done in the FPU or the SSE unit -- the FPU uses internally higher precision.) And to make things worse: floating point results MIGHT even depend on changes you perform at other places in the code (as this might influence the compilers optimization behaviour...) > >> > >> BTW, the original code was infinite looping until I replaced the old style: > >> > >> for (i=0; i<WHATEVER; i++) .. > >> > >> with i declared within the routine (i.e., function) with: > >> > >> for (int i=0; i<WHATEVER; i++) ... This part is really strange however: Both should give the same behaviour. The only reasons I can imagine without knowing the code: - the "i" in old-style syntax was no "int", but a different (e.g. smaller) type? - you have a subtle bug in the code which changes the "old-style i" from the loop (e.g. by an invalid pointer): your changed line will generate a NEW variable "i", which is valid during the loop only and is probably stored at another address in memory - there is a bug in gcc Can you supply a (small) working example which reproduces the problem? Axel
