c++ - 64 bit floating point porting issues

Question

I'm porting my application from 32 bit to 64 bit. Currently, the code compiles under both architectures, but the results are different. For various reasons, I'm using floats instead of doubles. I assume that there is some implicit upconverting from float to double happening on one machine and not the other. Is there a way to control for this, or specific gotchas I should be looking for?

edited to add:

32 bit platform

 gcc (GCC) 4.1.2 20070925 (Red Hat 4.1.2-33)
 Dual-Core AMD Opteron(tm) Processor 2218 HE

64 bit platform

 gcc (Ubuntu 4.3.3-5ubuntu4) 4.3.3
 Intel(R) Xeon(R) CPU

Applying the -mfpmath=387 helps somewhat, after 1 iteration of the algorithm the values are the same, but beyond that they fall out of sync again.

I should also add that my concern isn't that the results aren't identical, it's that porting to a 64 bit platform has uncovered a 32 bit dependency of which I was not aware.

Answer 1

Your compiler is probably using SSE opcodes to do most of its floating point arithmetic on the 64 bit platform assuming x86-64, whereas for compatibility reasons it probably used the FPU before for a lot of its operations.

SSE opcodes offer a lot more registers and consistency (values always remain 32 bits or 64 bits in size), while the FPU uses 80 bit intermediate values when possible. So you were most likely benefitting from this improved intermediate precision before. (Note the extra precision can cause inconsistent results like x == y but cos(x) != cos (y) depending on how far apart the computations occur!)

You may try to use -mfpmath=387 for your 64 bit version since you are compiling with gcc and see if your results match your 32 bit results to help narrow this down.