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FFmpeg School of Assembly Language

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Re: FFmpeg School of Assembly Language

#121

Post by zbobet2012 »

computerbuster wrote: post #85

Another resource on the same topic: https://blogs.gnome.org/rbultje/2017/07/14/writing-x86-simd-... As I'm seeing in the comments here, the usefulness of handwritten SIMD ranges from "totally unclear" to "mission critical". I'm seeing a lot on the "totally unclear" side, but not as much on the "mission critical", so I'll talk a bit about that. FFmpeg is a pretty clear use case because of how often it is used, but I t…

So on point. We do _a lot_ of hand written SIMD on the other side (encoders) as well for similar reasons. In addition on the encoder side it's often necessary to "structure" the problem so you can perform things like early elimination of loops, and especially loads. Compilers simply can not generate autovectorized code that does those kinds of things.

Re: FFmpeg School of Assembly Language

#123

Post by mobiledev2014 »

Daniel_Van_Zant wrote: post #50

I'm curious from anyone who has done it. Is there any "pleasure" to be had in learning or implementing assembly (like there is for LISP or RISC-V) or is it something you learn and implement because you want to do something else (like learning COBOL if you need to work with certain kinds of systems). It has always piqued my interest but I don't have a good reason in my day-to-day job to get into it. Wondering if it is…

Given there’s a mini genre of games that emulate using assembly to solve puzzles the answer is clearly yes. Not sure if any of them teach a real language.

The most popular are the Zachtronics games and Tomorrow Corp games. They’re so so good!

Re: FFmpeg School of Assembly Language

#125

Post by adamnemecek »

beebaween wrote: post #124

I'm kind of stunned we haven't gotten something better / more rust based than ffmpeg? Especially curious given the advent of apple metal etc. Does anyone have recommendations?

Why? It's a Herculean effort. It took it was 28 years between the creation of C and FFMPEG, so if there still is not a replacement by 2038, then your complaint is justified.

Re: FFmpeg School of Assembly Language

#126

Post by qingcharles »

kierank wrote: post #117

I am the author of these lessons. Ask me anything.

As someone who wrote x86 optimization code professionally in the 90s, do we need to do this manually still in 2025?

Can we not just write tests and have some LLM try 10,000 different algorithms and profile the results?

Or is an LLM unlikely to find the optimal solution even with 10,000 random seeds?

Just asking. Optimizing x86 by hand isn't the easiest, because to think through it you start to have to try and fit all the registers in your mind and work through the combinations. Also you need to know how long each instruction combination will take; and some of these instructions have weird edge cases that take vastly longer or quicker to run that is hard for a human to take into account.

Re: FFmpeg School of Assembly Language

#128

Post by qingcharles »

cnt-dracula wrote: post #120
kierank wrote: post #117

I am the author of these lessons. Ask me anything.

Hi, thanks for your work! I have a question, as someone who can just about read assembly but still do not intuitively understand how to write or decompose ideas to utilise assembly, do you have any suggestions to learn / improve this? As in, at what point would someone realise this thing can be sped up by using assembly? If one found a function that would be really performant in assembly how do you go about writing i…

You're looking for the tiniest blocks of code that are run an exceptional number of times.

For instance, I used to work on graphics renderers. You'd find the bit that was called the most (writing lines of pixels to the screen) and try to jiggle the order of the instructions to decrease the number of cycles used to move X bits from system RAM to graphics RAM.

When I was doing it, branching (usually checking an exit condition on a loop) was the biggest performance killer. The CPU couldn't queue up instructions past the check because it didn't know whether it was going to go true or false until it got there.

Re: FFmpeg School of Assembly Language

#129

Post by dundarious »

computerbuster wrote: post #85

Another resource on the same topic: https://blogs.gnome.org/rbultje/2017/07/14/writing-x86-simd-... As I'm seeing in the comments here, the usefulness of handwritten SIMD ranges from "totally unclear" to "mission critical". I'm seeing a lot on the "totally unclear" side, but not as much on the "mission critical", so I'll talk a bit about that. FFmpeg is a pretty clear use case because of how often it is used, but I t…

What does Zig offer in the way of builtin SIMD support, beyond overloads for trivial arithmetic operations? 90% of the utility of SIMD is outside of those types of simple operations. I like Zig, but my understanding is you have to reach for CPU specific builtins for the vast majority of cases, just like in C/C++.

GCC and Clang support the vector_size attribute and overloaded arithmetic operators on those "vectorized" types, and a LOT more besides -- in fact, that's how intrinsics like _mm256_mul_ps are implemented: `#define _mm256_mul_ps(a,b) (__m256)((v8sf)(a) * (v8sf)(b))`. The utility of all of that is much, much greater than what's available in Zig.

Re: FFmpeg School of Assembly Language

#130

Post by qingcharles »

qingcharles wrote: post #126
kierank wrote: post #117

I am the author of these lessons. Ask me anything.

As someone who wrote x86 optimization code professionally in the 90s, do we need to do this manually still in 2025? Can we not just write tests and have some LLM try 10,000 different algorithms and profile the results? Or is an LLM unlikely to find the optimal solution even with 10,000 random seeds? Just asking. Optimizing x86 by hand isn't the easiest, because to think through it you start to have to try and fit all…

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