4 ms·

Roll my eyes every time I see a "Considered Harmful" headline. As for SIMD, it's a huge benefit when used in the right context. I applied it image processing a

by NL807 8y ago

Roll my eyes every time I see a "Considered Harmful" headline. As for SIMD, it's a huge benefit when used in the right context. I applied it image processing and video compression algorithms in the past, with significant performance gains.

DerekL 8y ago
In your opinion, how does SIMD compare with “vector architectures” described in the article?
tom_mellior 8y ago
> Roll my eyes every time I see a "Considered Harmful" headline. Me too. > As for SIMD, it's a huge benefit when used in the right context. Sure, but as the article points out, that benefit could be even huger when used on a "proper" vector architecture with veeeery wide vector registers that do not also double as not-very-wide scalar registers. "The SIMD instructions execute 10 to 20 times more instructions than RV32V because each SIMD loop does only 2 or 4 elements instead of 64 in the vector case." I think adding SIMD instructions to x86 was a good trade-off at the time, but I also think the authors are correct that new ISAs designed now are better off with a vector architecture like they propose. In the end it's apples vs. oranges because the two contexts are not comparable.
- lukego 8y ago
  I haven't written code for a vector architecture yet but, wow, they look so much nicer to program than SIMD on casual inspection.
- dkersten 8y ago
  I feel like even though full vector architecture might perform a lot better, the use cases may be much narrower than SIMD, especially on typical desktop or server (web applications etc, not scientific computing, deep learning or image processing -- many of which are already vectorised on GPUs) workloads. As others have mentioned, SIMD allows you to do a little bit of vectorisation in an otherwise non-vector workload, or use it for the wider registers or whatever. I don't know enough about it personally to be able to judge either way, though. I just know that I've attempted to vectorise some hobby game code for fun a few times and typically found it much harder to achieve than it first seemed, even though the data seemed trivially vectorisable at first. Perhaps that's just lack of experience.
vermilingua 8y ago
https://meyerweb.com/eric/comment/chech.html https://meyerweb.com/eric/comment/chech.html
- pjscott 8y ago
  This is one of the few times when a Considered Harmful article deserves the title. Did you see the RISC-V assembly code? Holy shit! It's so much nicer than every other SIMD ISA I've ever used! So much easier to program, so much easier to write compilers for -- and it should also be easier for CPU designers to handle efficiently. What's not to like?
  - richardwhiuk 8y ago
    > should also be easier for CPU designers to handle efficiently I'm always dubious of claims like this. I think in general the architecture would need to turn it back into the highest SIMD code it supports at instruction decode.
bartread 8y ago
> Roll my eyes every time I see a "Considered Harmful" headline. Same. Please, when you write a title for your article, be specific about what you think is wrong. "X Considered Harmful" is lazy writing and devalues any substantive argument you make because it's become a cliche to the point where it's almost anti-clickbait. GUILTY ADMISSION: a related trap I started falling into when writing content for work (email, documents, whatever) and I was in a hurry for a subject line or title was "Thoughts on X". That's right, "thoughts": I communed with the creator and distilled them from on high. Yeah, ain't nobody got time to read that.
kazinator 8y ago
"Harmful" doesn't mean "so dead in the water that it doesn't actually have performance gains". Intel wouldn't have integrated SIMD if it didn't work at all; that's not the criticism. Rather, the claim is that the performance gains are not well justified by the technical debt they bring to the architecture. The performance gains are not as good as they could be with the vector approach, which has better dynamic and static code density, doesn't horribly proliferate the instruction set, and allows for tuning without recompilation of code to use different instructions.
- electrograv 8y ago
  For those who don’t get the title reference: ”[Programming Pattern] Considered Harmful” is a title meme that began in 1968 — over fifty years ago — and still going strong today! It all started with the famous Edsger Dijkstra paper titled “Go To Statement Considered Harmful” [1], which lead to an endless series of subsequent papers later patterned after its title [2]. I agree with the sentiment that this title pattern is overused currently, to the point of being cliche — with perhaps a bit of presumptuousness as well, due to the implicit suggestion that the author’s claim of “Considered Harmful” will withstand the test of time as well as Dijkstra’s paper (though perhaps I’m reading too much into it, in this case). [1] https://dl.acm.org/citation.cfm?doid=362929.362947 https://dl.acm.org/citation.cfm?doid=362929.362947 [2] https://en.m.wikipedia.org/wiki/Considered_harmful https://en.m.wikipedia.org/wiki/Considered_harmful

by NL807 8y ago

DerekL 8y ago

In your opinion, how does SIMD compare with “vector architectures” described in the article?

tom_mellior 8y ago

> Roll my eyes every time I see a "Considered Harmful" headline. Me too. > As for SIMD, it's a huge benefit when used in the right context. Sure, but as the article points out, that benefit could be even huger when used on a "proper" vector architecture with veeeery wide vector registers that do not also double as not-very-wide scalar registers. "The SIMD instructions execute 10 to 20 times more instructions than RV32V because each SIMD loop does only 2 or 4 elements instead of 64 in the vector case." I think adding SIMD instructions to x86 was a good trade-off at the time, but I also think the authors are correct that new ISAs designed now are better off with a vector architecture like they propose. In the end it's apples vs. oranges because the two contexts are not comparable.

lukego 8y ago
I haven't written code for a vector architecture yet but, wow, they look so much nicer to program than SIMD on casual inspection.
dkersten 8y ago
I feel like even though full vector architecture might perform a lot better, the use cases may be much narrower than SIMD, especially on typical desktop or server (web applications etc, not scientific computing, deep learning or image processing -- many of which are already vectorised on GPUs) workloads. As others have mentioned, SIMD allows you to do a little bit of vectorisation in an otherwise non-vector workload, or use it for the wider registers or whatever. I don't know enough about it personally to be able to judge either way, though. I just know that I've attempted to vectorise some hobby game code for fun a few times and typically found it much harder to achieve than it first seemed, even though the data seemed trivially vectorisable at first. Perhaps that's just lack of experience.

vermilingua 8y ago

https://meyerweb.com/eric/comment/chech.html https://meyerweb.com/eric/comment/chech.html

pjscott 8y ago
This is one of the few times when a Considered Harmful article deserves the title. Did you see the RISC-V assembly code? Holy shit! It's so much nicer than every other SIMD ISA I've ever used! So much easier to program, so much easier to write compilers for -- and it should also be easier for CPU designers to handle efficiently. What's not to like?
- richardwhiuk 8y ago
  > should also be easier for CPU designers to handle efficiently I'm always dubious of claims like this. I think in general the architecture would need to turn it back into the highest SIMD code it supports at instruction decode.

bartread 8y ago

> Roll my eyes every time I see a "Considered Harmful" headline. Same. Please, when you write a title for your article, be specific about what you think is wrong. "X Considered Harmful" is lazy writing and devalues any substantive argument you make because it's become a cliche to the point where it's almost anti-clickbait. GUILTY ADMISSION: a related trap I started falling into when writing content for work (email, documents, whatever) and I was in a hurry for a subject line or title was "Thoughts on X". That's right, "thoughts": I communed with the creator and distilled them from on high. Yeah, ain't nobody got time to read that.

kazinator 8y ago

"Harmful" doesn't mean "so dead in the water that it doesn't actually have performance gains". Intel wouldn't have integrated SIMD if it didn't work at all; that's not the criticism. Rather, the claim is that the performance gains are not well justified by the technical debt they bring to the architecture. The performance gains are not as good as they could be with the vector approach, which has better dynamic and static code density, doesn't horribly proliferate the instruction set, and allows for tuning without recompilation of code to use different instructions.

electrograv 8y ago
For those who don’t get the title reference: ”[Programming Pattern] Considered Harmful” is a title meme that began in 1968 — over fifty years ago — and still going strong today! It all started with the famous Edsger Dijkstra paper titled “Go To Statement Considered Harmful” [1], which lead to an endless series of subsequent papers later patterned after its title [2]. I agree with the sentiment that this title pattern is overused currently, to the point of being cliche — with perhaps a bit of presumptuousness as well, due to the implicit suggestion that the author’s claim of “Considered Harmful” will withstand the test of time as well as Dijkstra’s paper (though perhaps I’m reading too much into it, in this case). [1] https://dl.acm.org/citation.cfm?doid=362929.362947 https://dl.acm.org/citation.cfm?doid=362929.362947 [2] https://en.m.wikipedia.org/wiki/Considered_harmful https://en.m.wikipedia.org/wiki/Considered_harmful