Is it possible to use float and double without setting FPU?

[mrjbom]

This is a very stupid question, but I thought I'd ask it nonetheless.
Now in my early initialization code I need to use double for calculations, but I'm not sure I can perform it as it seems to require the FPU to be preconfigured, is this true?

[iansjack]

https://wiki.osdev.org/FPU

But why would you need to use floating-point calculations when booting an operating system?

[eekee]

We can help convert your calculations to integer ones, if you'd like.

[Octocontrabass]

If you really need floating-point math, there are ways to do it without using the FPU.

But you probably don't need floating-point math.

[mrjbom]

But why would you need to use floating-point calculations when booting an operating system?

To calculate when I should remap the hash map.

Code: Select all

m->remap = (uint32_t)(m->capacity * ((double)m->load_fac / 100));

For example, m->capacity is 8 and m->load_fac = 75, then, I will need to increase the size of the hash map when the number of elements in it reaches 6.
Specifically this calculation I could replace with something like:

Code: Select all

m->remap = m->capacity / 4 * 3

But can it be done any better? So that I can have different load_fac values, for example?

[iansjack]

The integer calculation

Code: Select all

m->remap = (m->capacity * m->load_fac + 99) / 100

(if you want to round up) produces the same result. If you want to round down just use

Code: Select all

m->remap = (m->capacity*m->load->fac) / 100

[iProgramInCpp]

Specifically this calculation I could replace with something like:

Code: Select all

m->remap = m->capacity / 4 * 3

This would work better as a replacement for what I quoted above.

Code: Select all

m->remap = m->capacity * 3 / 4

[mrjbom]

Specifically this calculation I could replace with something like:

Code: Select all

m->remap = m->capacity / 4 * 3

This would work better as a replacement for what I quoted above.

Code: Select all

m->remap = m->capacity * 3 / 4

Why?

[iansjack]

You need to do the multiplication before the division (if using integer arithmetic) to get the correct result. And it’s best to use parentheses to ensure the correct order.

Code: Select all

8 * (75 / 100) = 0

whereas

Code: Select all

(8 * 75) / 100 = 6

The latter is obviously the result you want.

You’re looking for an integer result, so you don’t need floating-point calculations.

[mrjbom]

You need to do the multiplication before the division (if using integer arithmetic) to get the correct result. And it’s best to use parentheses to ensure the correct order.

Code: Select all

8 * (75 / 100) = 0

whereas

Code: Select all

(8 * 75) / 100 = 6

The latter is obviously the result you want.

You’re looking for an integer result, so you don’t need floating-point calculations.

I understand this.
He said that I should replace

Code: Select all

m->remap = m->capacity / 4 * 3

with

Code: Select all

m->remap = m->capacity * 3 * 4

which I did not understand.

[thewrongchristian]

You need to do the multiplication before the division (if using integer arithmetic) to get the correct result. And it’s best to use parentheses to ensure the correct order.

Code: Select all

8 * (75 / 100) = 0

whereas

Code: Select all

(8 * 75) / 100 = 6

The latter is obviously the result you want.

You’re looking for an integer result, so you don’t need floating-point calculations.

I understand this.
He said that I should replace

Code: Select all

m->remap = m->capacity / 4 * 3

with

Code: Select all

m->remap = m->capacity * 3 * 4

which I did not understand.

No.

The suggested code was:

Code: Select all

m->remap = m->capacity * 3 / 4

The reason it was suggested is because doing the division by 4 first will lose you precision. If you multiply by 3 first, you lose no precision in the intermediate result before dividing by 4.

The other good thing about the code is that the optimiser will turn this all into shifts and adds, so it'll be blazing fast.