64 bit floating point porting issues
64-bit, c, c++, floating-point, portability
Solution
There is no inherent need for floats and doubles to behave differently between 32-bit and 64-bit code but frequently they do. The answer to your question is going to be platform and compiler specific so you need to say what platform you are porting from and what platform you are porting to.
On intel x86 platforms 32-bit code often uses the x87 co-processor instruction set and floating-point register stack for maximum compatibility whereas on amb64/x86_64 platforms, the SSE* instructions and xmm* registers and are often used instead. These have different precision characteristics.
Post edit:
Given your platform, you might want to consider trying the -mfpmath=387 (the default for i386 gcc) on your x86_64 build to see if this explains the differing results. You may also want to look at the settings for all the -fmath-* compiler switches to ensure that they match what you want in both builds.
Problem
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.