Floating point mismatch between Visual Studio 2008 and 2013

Question

After upgrading C++ project to Visual studio 2013, the result of the program has changed because of different floating point behavior of the new VC compiler. The floating mo

Answer 1

The behavior is correct, the float type can store only 7 significant digits. The rest are just random noise. You need to fix the bug in your code, you are either displaying too many digits, thus revealing the random noise to a human, or your math model is losing too many significant digits and you should be using double instead.

There was a significant change in VS2012 that affect the appearance of the noise digits. Part of the code generator changes that implement auto-vectorization. The 32-bit compiler traditionally used the FPU for calculations. Which is very notorious for producing different random noise, calculations are performed with an 80-bit intermediate format and get truncated when stored back to memory. The exact moment when this truncation occurs can be unpredictable due to optimizer choices, thus producing different random noise.

The code generator, like it already did for 64-bit code, now uses SSE2 instructions instead of FPU instructions. Which produce more consistent results that are not affected by the code optimizer choices, the 80-bit intermediate format is no longer used. A backgrounder on the trouble with the FPU is available in this answer.

This will be the behavior going forward, the FPU will never come back again. Adjust your expectations accordingly, there's a "new normal". And fix the bugs.