Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

FFmpeg School of Assembly Language

github.com

Post reply on HN
Print view View original

221–226 of 226 posts

Re: FFmpeg School of Assembly Language

#221

Post by tengwar2 »

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…

Depends on whether you have a suitable problem, and the problem domains have shrunk greatly over the last 40 years. I've used it for bit-twiddling in real-time data acquisition; for implementing a GUI (on OS/2 1.0, which didn't come with one) and for acquiring a handy character set for the same from DOS; for accessing some obscure features of the PC architecture (thanks, Thom Hogan); for trying unsuccessfully to access paged screen memory with an RSX on the Z80 based Amstrad; and for successfully implementing an RSX to quash high-bit characters from the keyboard on the same to deal with an editor which had an array of only 128 characters for deciding what to do with input.

All of them apart from the screen memory thing were fun, but the only one which could be useful these days is the bit-twiddling. All the rest have been made obsolete by improved operating systems, so that the domain of useful assembler programs shrinks ever further. OTOH, debugging them is vastly easier than in the old days where all you got was random lines drawn across your screen as the system crashed, and you couldn't even single-step because you had to bank-switch out.

Re: FFmpeg School of Assembly Language

#222

Post by weebull »

janwas wrote: post #193
cornstalks wrote: post #149

Earlier quoted context omitted.

One of the fun things about dav1d is that since it’s written in assembly, they can use their own calling convention. And it can differ from method to method, so they have very few stack stores and loads compared to what a compiler will generate following normal platform calling conventions.

I'm curious why there are even function calls in time-critical code, shouldn't just about everything be inlined there? And if it's not time-critical, why are we interested in the savings from a custom calling convention?

Cache misses hurt.

Re: FFmpeg School of Assembly Language

#223

Post by Daniel_Van_Zant »

msaltz wrote: post #56
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…

I did the first 27 chapters of this tutorial just because I was interested in learning more and it was thoroughly enjoyable: https://mariokartwii.com/armv8/ I actually quite like coding in assembly now (though I haven’t done much more than the tutorial, just made an array library that I could call from C). I think it’s so fun because at that level there’s very little magic left - you’re really saying exactly what sho…

This looks to be very cool will check it out. Wild to see it on a Mario Kart Wii Site, but I guess modders/hackers are one of the groups of people who still need to work with assembly frequently.

Re: FFmpeg School of Assembly Language

#224

Post by Daniel_Van_Zant »

brown wrote: post #60
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…

Learning assembly was profound for me, not because I've used it (I haven't in 30 years of coding), but because it completed the picture - from transistors to logic gates to CPU architecture to high-level programming. That moment when you understand how it all fits together is worth the effort, even if you never write assembly professionally.

This out of everything, convvinced me. The more I get the "full picture" the more I appreciate what a wondrous thing computers are. I've learned all the way down to Forth/C and from the bottom up to programming FPGAs with Verilog so Assembly may be just what I need to finally close that last gap.

Re: FFmpeg School of Assembly Language

#225

Post by astrange »

screcth wrote: post #192
astrange wrote: post #111

Earlier quoted context omitted.

There's a difference because audio processing is often "massively parallel", or at least like 1024 samples at once, but in video codecs operations could be only 4 pixels at once and you have to stretch to find extra things to feed the SIMD operations.

Can you use the remaining SIMD lanes for processing independent data streams? Think encoding or decoding non-overlapping parts of a video.

So, you can't necessarily do that because video is compression, and compression means not predictable. (If it's predictable it's not compressed well enough.)

That means you have to stick to inside the current block. But there are some tricks; like for an IDCT there's a previous stage where you can rearrange the output memory elements for free, so you can shuffle things as you need to fit them into vectors.

Post reply on HN