Fast updates

11th April 2023

We regret the error…

From last time, there was indeed no performance problem with adding a (non-pointer) global to hold state. The problem was that I mashed another separate change into that work which accidentally turned the heap into .bss data. And having the loader have a whole heap to zero out before starting main() is not the best way to improve performance.

With that egg wiped off my face, onwards!

Compile timing

Once I got the C JIT to get to the main prompt of sqlite3, I wanted to check compile times vs. the no-opt build in clang and cl.


c:\src\dyibicc>timavg dyibicc shell.c sqlite3.c
avg = 0m1.722s

clang 15 -O0

c:\src\dyibicc>timavg clang -O0 shell.c sqlite3.c
avg = 0m2.868s

msvc 2022 /Od

c:\src\dyibicc>timavg cl /Od /nologo shell.c sqlite3.c
avg = 0m0.850s

cl is very quick!1 A quick profile and a few small tweaks to string processing got dyibicc down to avg = 0m1.478s, but there’s still a big gap there to the no-opt cl time.

I went off on a tangent setting up profilers and Deciding What Must Be Done to be faster than cl, but… that’s kind of boring when you don’t have an actual real target to optimize. sqlite3 is a good test case, but it’s weird (two enormous files), which stresses the tokenizer and parser, but not e.g. include path searching. I tried generating other test cases that were the opposite (many files, many includes, etc.) and those showed different bottlenecks. But short of “optimize everything”, none of it seemed especially bad, or in obvious need of fixing first.

Faster updates instead of faster compiles

So instead, I decided to continue on the embeddability. With the compiler’s state mostly bundled up successfully, I took another go at longjmping out of error conditions. There are undoubtedly corner case footguns that will get me later, but it actually seems to work well in practice after a bit more debugging.

Once that seemed to be working, I made a small shell app that combines Raylib with dyibicc, and bundles Neovim2. Raylib is a programmer-centric game/graphics library, in that there’s no level-editor-type thing, there’s just a big header of useful functions that you write code against.

The test shell (Rdy) starts by creating a top-level window via Raylib. It first does a full compile of all the files in the project, and then hooks up to the Neovim instance via RPC, which has been launched with the project’s C files.

Because they’re connected over RPC, all buffer changes can be sent directly to the compiler over RPC and there’s no need to even write files to disk. I actually started with normal file editing, and then having the shell watch for updates. But it was a bit tedious to have to constantly :wa. There’s also a lot of messy states that files can be in while the editor is (re)writing them: non-existent, zero bytes, locked, etc. so RPC avoids all that.

Every frame, the shell just services RPC to get updates to pass to the embedded compiler, starts drawing, calls the dyibicc C code entry point (if there wasn’t a compile error), and then page flips and repeats. It feels pretty slick when the running C code continuously matches what’s in the editor.

The question that then comes up is: What does it really mean to do a “live update” of C code?

It seems clear that when you change a function, you want the new one to be linked in, and further, that you want all callers that were calling the old implementation of the function to now call the new one. (You can sort of imagine where you might not want that, but I think it would get too confusing even if it were to be supported.)

Things in the data segment get less clear. If data segment values were reinitialized, then you’d effectively be restarting the program, so it seems that you mostly don’t want that.

But! You do also want to be able to tweak e.g. string constants, so things in .rodata/.rdata need to be updated to their new values. This wasn’t entirely obvious, as dyibicc inherited chibicc’s lack of distinguishing .rodata data from .data data. They’re somewhat teased apart now, but there’s likely more to do there.

It becomes fuzzy and requires understanding user-intent if you e.g. have an array of function pointers that’s globally initialized. Specifically, a reference from .data into .text, or to put it another way from “variable” into “constant”. In the current implementation, those will not update properly. When a new implementation of a function is linked in, and the old one removed, the new function will be at a different address. But the random void* stashed away in an array no longer has any real association with that function (at least by name) so the array will end up forever pointing to the first implementation of the function.

We could try keeping old function addresses around and patch them to start with a jmp to the new implementation, but I don’t love the never-garbage-collected aspect of that. There’s no technical reason why it’s any harder to update the array of function pointers, so I think the simplest solution would be to require an attribute/pragma on the object to indicate to the runtime that it should be refreshed. const might be enough of a “tag” in some cases too.

In summary, when relinking: Constants change, variables don’t. Which sounds funny out loud, but I think it makes sense.

An unscripted demo

That wall of text doesn’t really make it seem that cool, but it kind of is! I thought I would try to record a short-but-captivating video, because seeing it update in real time is the neat thing.

But as I’m lacking Streamer Star Charisma and snazzy video production, we instead have to settle for a video of me mumbling, while typing slowly and making typos, discovering bugs, and forgetting what I was going to talk about. But maybe you can kind of see what’s going on and how it could be neat.

(You’ll need to open it on YT and make sure it’s set to 1080p and fullscreen or you’re definitely not going to be able to see what’s going on.)

  1. And even while printing lots of unnecessary console spam! hashtag always annoying 

  2. Going straight to vi for the IDE strenously avoids popularity! 

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