[Const-me]

> Can you actually be faster than C?

Sometimes. I know 2 reasons.

1. In some higher-level languages, some problems can be efficiently solved with code generation. For instance, you can take some of the input data, generate code from that, then run the generated code processing the rest of the input data. Examples of the mechanism include Lisp macros, or .NET shenanigans like System.Reflection.Emit or System.Linq.Expressions.

It’s hard to generate good machine code. Possible to do in C, but you gonna need to embed C compiler for that. They are extremely complex, and were not designed for the use case e.g. relatively slow (designed to run offline on fast developer’s computers, or on even faster build servers).

If your problem is a good fit for that approach, C# can actually be faster than C.

Parsing JSON is a good example. When your goal is not just parsing the syntax, but also populating structures with the data from the JSON, good C# JSON parsers gonna runtime-generate serialization code by reflecting the structure types being populated. For an isolated program, technically you can generate equivalent C code offline. But if you’re making a general-purpose JSON serialization library that’s borderline impossible to achieve in C, need reflection and code generation.

2. Modern computers are heterogenous, they have 2 chips computing stuff, CPU and GPU. Even when these pieces are on the same chip and using the same RAM like Intel’s UHD graphics or AMD’s APUs, GPU can be still faster for some problems. Not only because more GFlops (that’s not necessarily true for integrated graphics), also because GPUs have better strategy for dealing with RAM latency. They switch threads instead of waiting for the data to arrive. CPU cores only run 1-2 hardware threads each i.e. limited in that regard.

That’s how for some problems HLSL or OpenCL can actually be faster than C.

[jcelerier]

> They are extremely complex, and were not designed for the use case e.g. relatively slow (designed to run offline on fast developer’s computers, or on even faster build servers).

in 2021 bundling clang along with your program is actually reasonable - if you are compiling small functions without two tons of headers it's measured in milliseconds.

[Const-me]

I never tried to, but I think integration of runtime-generated native code gonna cause overhead.

Where do you place these functions once compiled? Into a separate DLL/each?

In .NET it’s quite easy to generate code that calls manually written functions, or access data provided by manually-written stuff. JIT runtime doesn’t treat generated code as something special, e.g. may inline calls across runtime-generated and manually written pieces of the program. With clang you gonna need a layer of indirection to integrate, with function pointers and such.

[jcelerier]

LLVM can just put the compiled code at some place in your ram and then you can just execute it

[Const-me]

Processors need addresses of things. Look at the following code https://godbolt.org/z/PYEqKn note the function uses another symbol, “func.counter”.

Shared libraries include relocation tables https://en.wikipedia.org/wiki/Relocation_%28computing%29 with all code locations which needs patching. That’s how the OSes can load them into arbitrary locations in memory and the code will still work.

[jcelerier]

... yes ? https://github.com/weliveindetail/JitFromScratch

[Const-me]

Interesting, I didn’t know llvm is that flexible.

Still, LLVM is a huge dependency to redistribute. And probably has many points of failure. For instance, I expect you gonna need to watch for INCLUDE and PATH environment variables when using that thing.

In .NET everything is in the standard library. The generated code is even platform-independent, for no additional overhead. Here’s couple non-trivial examples: https://github.com/Const-me/ComLightInterop/blob/master/ComL... https://github.com/Const-me/ComLightInterop/blob/master/ComL...

[namibj]

I'll give Terra[0] as an example for something relatively high-level that uses LLVM as a JIT. It can also be used as an AoT compiler with fancy macro stuff in place of the C preprocessor.

[0]: http://terralang.org/