Lua floating point operations

Question

I run Lua on a CPU without dedicated floating point HW, depending on SW emulation.

From luaopt.h I can see that some macros are set to double, but it does not clearly state when floats are used and its a little hard to track it.

If my script does simple stuff like:

a=0
a=a+1
for...

Would that involve a floating point operations at any level?

If no that's fine, but what is then the benefit to change macros to long?

(I tried of course but did not work....)

RBerteig RBerteig · Accepted Answer · 2014-01-06T23:24:20

All numeric operations in Lua are performed (according to the default configuration) in floating point. There is no distinction made between floating point and integer, all values are simply numbers.

The actual C type used to store a Lua number is set in luaconf.h, and it is both allowed and even practical to change that to a suitable integral type. You start by changing LUA_NUMBER from double to int, long, or perhaps ptrdiff_t. Then you will find you need to tweak the related macros that control the conversions between strings and numbers. And, of course, you will likely need to eliminate most or all of the base math library since math.sin() and its friends and neighbors are not particularly useful over integers.

The result will be a Lua interpreter where all numbers are integers. The language will still allow you to type 3.14, but it will be stored as 3. Your code will likely not be completely portable to a Lua interpreter built with the standard configuration since a huge amount of Lua code casually assumes that floating point arithmetic is permitted, and remember that your compiled byte code will definitely not be compatible since byte code will store numbers as LUA_NUMBER.

Lua floating point operations

2 Answers