Why do you program in assembly?

Question

I have a question for all the hardcore low level hackers out there. I ran across this sentence in a blog. I don't really think the source matters (it's Haack if you really care) because it seems to be a common statement.

For example, many modern 3-D Games have their high performance core engine written in C++ and Assembly.

As far as the assembly goes - is the code written in assembly because you don't want a compiler emitting extra instructions or using excessive bytes, or are you using better algorithms that you can't express in C (or can't express without the compiler mussing them up)?

I completely get that it's important to understand the low-level stuff. I just want to understand the why program in assembly after you do understand it.

Answer 1

I think a lot of game developers would be surprised at this bit of information.

Most games I know of use as little assembly as at all possible. In some cases none at all, and at worst, one or two loops or functions.

That quote is over-generalized, and nowhere near as true as it was a decade ago.

But hey, mere facts shouldn't hinder a true hacker's crusade in favor of assembly. ;)

Answer 2

It doesn't seem to be mentioned, so I thought I'd add it: in modern games development, I think at least some of the assembly being written isn't for the CPU at all. It's for the GPU, in the form of shader programs.

This might be needed for all sorts of reasons, sometimes simply because whatever higher-level shading language used doesn't allow the exact operation to be expressed in the exact number of instructions wanted, to fit some size-constraint, speed, or any combination. Just as usual with assembly-language programming, I guess.

Answer 3

There is one aspect of assembler programming which others have not mentioned - the feeling of satisfaction you get knowing that every single byte in an application is the result of your own effort, not the compiler's. I wouldn't for a second want to go back to writing whole apps in assembler as I used to do in the early 80s, but I do miss that feeling sometimes...

Answer 4

The only assembler coding I continue to do is for embedded hardware with scant resources. As leander mentions, assembly is still well suited to ISRs where the code needs to be fast and well understood.

A secondary reason for me is to keep my knowledge of assembly functional. Being able to examine and understand the steps which the CPU is taking to do my bidding just feels good.

Answer 5

SSE code works better in assembly than compiler intrinsics, at least in MSVC. (i.e. does not create extra copies of data )