Re:Don't use assembly unless you're an expert
by dankoss on Friday April 04, 2014 @10:18AM
(#7531534)
I also do DSP development and looking at the x86 architecture it almost seems pointless to try to predict what the processor is doing. Out of order execution? Register renaming? The only way I can be sure assembly is faster than generated C is by profiling, which isn't even exact anyway without isolating the operating system.
For us, latency and determinism are the most important requirements of any processor, and it's very hard to get that on x86 without writing your own OS. But that negates the entire advantage of using the x86 platform with so much available code written on top of other kernels.
Re:Don't use assembly unless you're an expert
by crazy1van on Friday April 04, 2014 @04:05PM
(#7533972)
This brings back memories of when I wrote a lot of DSP code. The architecture I was on had a decent C compiler, but it wouldn't correctly take advantage of some of the special instructions. By rewriting a viterbi decoder in assembly to take advantage of their special subtract-compare-branch instructions, I recall getting a huge performance increase. But the compiler did a decent job at optimizing for multiply-accumulate instructions especially if you gave it the right hints with pragmas.
Re:Don't use assembly unless you're an expert
by ballard on Saturday April 05, 2014 @10:54AM
(#7537224)
I wrote x86 assembly demos for fun in the golden age of asm between the eras of 8088 and p6... It became so much about putting random instructions here and there to get better performance, one almost needs like a QuickCheck-like sythensizing+benchmarking hybrid compiler that automagically mutates code to find what is actually fastest by genetic algo-like heuristics.
I write assembly for DSPs on a near-daily basis. Up until a few months ago there didn'nt even exist a C compiler for the target architecture.
Even when you write C code for an embedded platform that does have a decent C toolchain, you cannot truly understand what you're doing without spending a lot of time looking at the generated assembly, and writing some of it yourself.
However, this kind of architecture has nothing to do with the x86. RISC, no cache, in most cases no MMU (and rarely any DMA), an extremely simple and straightforward pipeline, etc. I've written assembly for several such architectures, but compared to them I find x86 assembly intimidating.