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FFmpeg School of Assembly Language

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Re: FFmpeg School of Assembly Language

#21

Post by lukaslalinsky »

imglorp wrote: post #18

Asm is 10x faster than C? That was definitely true at some point but is it still true today? Have compilers really stagnated so badly they can't come close to hand coded asm?

This is for heavily vectorized code, using every hack possible to fully utilize the CPU. Compilers are smart when it comes to normal code, but codecs are not really normal code. Not a ffmpeg programmer, but have some background dealing with audio.

Re: FFmpeg School of Assembly Language

#22

Post by 1propionyl »

imglorp wrote: post #18

Asm is 10x faster than C? That was definitely true at some point but is it still true today? Have compilers really stagnated so badly they can't come close to hand coded asm?

It's not a matter of compiler stagnation. The compiler simply isn't privy to the information the assembly author makes use of to inform their design.

Put more simply: a C compiler can't infer from a plain C implementation that you're trying to do certain mathematics that could alternately be expressed more efficiently with SIMD intrinsics. It doesn't have access to your knowledge about the mathematics you're trying to do.

There are also target specific considerations. A compiler is, necessarily, a general purpose compiler. Problems like resource (e.g. register) allocation are NP-complete (equivalent to knapsack) and very few people want their compiler to spend hours upon hours searching for the absolute most optimal (if indeed you can even know that statically...) asmgen.

Re: FFmpeg School of Assembly Language

#23

Post by warble »

imglorp wrote: post #18

Asm is 10x faster than C? That was definitely true at some point but is it still true today? Have compilers really stagnated so badly they can't come close to hand coded asm?

I highly doubt it's true. I can usually approach the same speed in C if I'm working with a familiar compiler. Sometimes I can do significantly better in assembly but it's rare.

I work on bare metal embedded systems though, so maybe there's some nuance when working with bigger OS libs?

Re: FFmpeg School of Assembly Language

#24

Post by 1propionyl »

jki275 wrote: post #20
imglorp wrote: post #18

Asm is 10x faster than C? That was definitely true at some point but is it still true today? Have compilers really stagnated so badly they can't come close to hand coded asm?

Probably some very niche things. I know I can't write ASM that's 10x better than C, but I wouldn't assume no one can.

It depends on what you're trying to do. I would in general only expect such substantial speedups when considering writing computation kernels (for audio, video, etc).

Compilers today are liable in most circumstances to know many more tricks than you do. Especially if you make use of hints (e.g. "this memory is almost always accessed sequentially", "this branch is almost never taken", etc) to guide it.

Re: FFmpeg School of Assembly Language

#25

Post by jsheard »

imglorp wrote: post #18

Asm is 10x faster than C? That was definitely true at some point but is it still true today? Have compilers really stagnated so badly they can't come close to hand coded asm?

C with intrinsics can get very close to straight assembly performance. The FFmpeg devs are somewhat infamously against intrinsics (IIRC they don't allow them in their codebase even if the performance is as good as equivalent assembly) but even by TFAs own estimates the difference between intrinsics and assembly is on the order of 10-15%.

You might see a 10x difference if you compare meticulously optimized assembly to naive C in cases where vectorization is possible but the compiler fails to capitalize on that, which is often, because auto-vectorization still mostly sucks beyond trivial cases. It's not really a surprise that expert code runs circles around naive code though.

Re: FFmpeg School of Assembly Language

#26

Post by epolanski »

imglorp wrote: post #18

Asm is 10x faster than C? That was definitely true at some point but is it still true today? Have compilers really stagnated so badly they can't come close to hand coded asm?

I remember a series of lectures from an Intel engineer that went into how difficult it was writing assembly code for x86. He basically stated that the number of cases you can really write code that is faster than what a compiler would do is close to none.

Essentially people think they are writing low level code, in reality that's not how CPUs interpret that code, so he explained how writing manual assembly kills performance pretty much always (at least on modern x86).

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