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Wed
17
Mar 2021
Floating-point numbers are a great invention in computer science, but they can also be tricky and troublesome to use correctly. I’ve written about them already by publishing Floating-Point Formats Cheatsheet and presentation “Pitfalls of floating-point numbers” (“Pułapki liczb zmiennoprzecinkowych” – the slides are in Polish). Last year I was preparing for a more extensive talk about this topic, but it got cancelled, like pretty much everything in these hard times of the COVID-19 pandemic. So in this post, I would like to approach this topic from a different angle.
A programmer can use floating-point numbers on different levels of understanding. A beginner would use them, trusting they are infinitely capable and precise, which can lead to problems. An intermediate programmer knows that they have some limitations, and so by using some good practices the problems can be avoided. An advanced programmer understands what is really going on inside these numbers and can use them with a full awareness of what to expect from them. This post may help you jump from step 2 to step 3. Commonly adopted good practices are called “myths” here, but they are actually just generalizations and simplifications. They can be useful for avoiding errors, unless you understand what is true and what is false about them on a deeper level.
1. They are not exact
It is not true that 2.0 + 2.0 can give 3.99999. It will always be 4.0. They are exact to the extent of their limited range and precision. If you assign a floating-point number some constant value, you can safely compare it with the same value later, even using the discouraged operator ==, as long as it is not a result of some calculations. Imprecisions don't come out of nowhere.
Instead of using integer loop iterator and converting it to float every time:
for(size_t i = 0; i < count; ++i)
{
float f = (float)i;
// Use f
}
You can do this, which will result in a much more efficient code:
for(float f = 0.f; f < (float)count; f += 1.f)
{
// Use f
}
It is true, however, that your numbers may not look exactly as expected because:
count in the example above was >16M, it would cause an infinite loop.64-bit “double”, however, represents integers exactly up to 9,007,199,254,740,992, so it should be enough for most applications. No wonder that some scripting languages do just fine while supporting only “double” floating-point numbers and no integers at all.
2. They are non-deterministic
It is not true that cosmic radiation will flip the least significant bit at random. Random number generators are also not involved. If you call the same function with your floating-point calculations with same input, you will get the same output. It is fully deterministic, like other computing. (Note: When old FPU instructions are generated rather than new SSE, this can be really non-deterministic and even a task switch may alter your numbers. See this tweet.)
It is true, however, that you may observe different results because:
I heard a story of a developer who tried to calculate hashes from the results of his floating-point calculations in a distributed system and discovered that records with what was supposed to be same data had different hashes on different machines.
I once had to investigate a user complaint about a following piece of shader code (in GLSL language). The user said that on AMD graphics cards for uv.x higher than 306 it always returns black color (zero).
vec4 fragColor = vec4(vec3(fract(sin(uv.x * 2300.0 * 12000.0))), 1.0);
I noticed that the value passed to sine function is very high. For uv.x = 306 it is 27,600,000. If we recall from math classes that sine cycles between -1 and 1 every 2*PI ≈ 6.283185 and we take into consideration that above 16,777,216 a 32-bit float cannot represent all integer numbers exactly, but start jumping every 2, then every 4 etc., we can conclude that we have not enough precision to know whether our result should be -1, 1, or anything in between. It is just undefined.
I then asked the user what is he trying to achieve with this code, as the result is totally random. He said it is indeed suppposed to be... a random number generator. The problem is that the result being always 0 is as valid as any other. The reason random numbers are generated on NVIDIA cards and not on AMD is that sine instruction on AMD GPU architectures actually has period of 1, not 2*PI. But it is still fully deterministic in regards to input value. It just returns different results between different platforms.
3. NaN and INF are indication of an error
It is true that if you don’t expect them, their appearance may indicate an error, either in your formulas or in input data (e.g. numbers very large, very small and close to zero, or just garbage binary data). It is also true that they can cause trouble as they propagate through calculations, e.g. every operation with NaN returns NaN.
However, it is not true that these special values are just a means of returning error or that they are not useful. They are perfectly valid special cases of the floating-point representation and have clearly defined behavior. For example, -INF is smaller and +INF is larger than any finite number. You can use this property to implement following function with a clearly documented interface:
#include <limits> // Finds and returns maximum number from given array.
// For empty array returns -INF.
float CalculateMax(const float* a, size_t count)
{
float max = -std::numeric_limits<float>::infinity();
for(size_t i = 0; i < count; ++i)
if(a[i] > max)
max = a[i];
return max;
}
Summary
As you can see, common beliefs about floating-point numbers - that they are not exact, non-deterministic, or that NaN and INF are an indication of an error, are some generalizations and simplifications that can help to avoid errors, but they don’t tell the full story. To really understand what's going on on a deeper level:
Update 2021-06-09: This article has been published as a guest post on C++ Stories and spawned an interesting discussion on Reddit that is worth reading.