4 ms·

I'd say that the theoretical ability to gang units together would be appealing. If you have four 128-bit packed SIMD, you must execute 4 different instructions

by hajile 2y ago

I'd say that the theoretical ability to gang units together would be appealing. If you have four 128-bit packed SIMD, you must execute 4 different instructions at once or the others go to waste. With SVE, you could (in theory) use all 4 as a single, very wide vector for common operations if there weren't a lot of instructions competing for execution ports. You could even dynamically allocate them based on expected vector size or amount of vector instructions coming down the pipeline. Additionally, adding two 2048-bit vectors using NEON (128-bit packed SIMD) would require 16 add instructions while SVE would require just one. That's a massive code size reduction which matters for I-cache and the frontend throughput.

dzaima 2y ago
You can't do 2048 bits of addition in one SVE instruction; not portably, at least (and definitely not on any existing hardware). While the maximum SVE register size is 2048 bits, the minimum is 128 bits, and the hardware chooses the supported register size, not the programmer. For portable SVE, your code needs to work for all of those widths, not just the smallest or largest. (of related note is RISC-V RVV, which allows you to group up to 8 registers together, allowing a minimum portable operation width of 128×8 = 1024 bits in a single instruction (and up to 65536×8 = 64KB for hypothetical crazy hardware with max VLEN), but SVE/SVE2 don't have any equivalent)
brigade 2y ago
A for() loop does the same thing at the cost of like 3 instructions. 4x128b has the flexibility that you don't need 512b wide operations on the same data to keep the ALUs fed. If you have 512b wide operations being split to 4x128b instructions, great, otherwise the massive OoOE window of modern chips can decode the next few loop iterations to keep the ALUs fed, or even pull instructions from a completely different kernel.
ribit 2y ago
I don't see how this would work out beneficially. Let's say your hardware can join 4x128b units as a virtual 512-bit SVE SIMD unit. This means you have to advertise VL as 512bit for reasons of consistency. Yes, you will save some entries in the reorder buffer if you encounter a single SVE instruction, but if the code contains independent SVE streams, you will be stalled. Moreso, not all operations will utilize all 512 register bits, so your occupancy might suffer. The only scenario I see this feature working out is if you are decode or reorder buffer limited. Neither is a problem for modern high-performance ARM cores. With x86, it might be a different story. From what I understand, AVX512 instructions can be quite large. Modern out-of-order cores are already good at superscalar execution, so why not let them do their job? 4x128b units give you much more flexibility and better execution granularity.
- janwas 2y ago
  On x86 at least, the cost of OoO is astonishing - more pJ per instruction dispatch than the operation itself. Amortizing that over more operations is the whole point of SIMD. I have not yet seen such data for Arm. That aside, see the "cmp" sibling thread for a major (4x penalty) downside to 4x128.
  - ribit 2y ago
    Yes, OoO is expensive — after all, that is the cost of performance. Very wide SIMD is great for energy efficiency if that is what your compute patterns require (there is a good reason why GPUs are in-order very wide SMT SIMD processors). Is this the best choice for a general-purpose CPU? That I am not so sure about. A CPU needs to be able to run all kinds of code. A single wide SIMD unit is great for some problems, but it won't deliver good performance if you need more flexibility. Could you point me to the "cmp" thread you mentioned? I don't know where to look for it.
    - janwas 2y ago
      I agree with you we do not only want "very wide SIMD", and it seems to me that 2x512-bit (Intel) or 4x256 (AMD) are actually a good middle ground. Sure, it's https://news.ycombinator.com/item?id=40465090 https://news.ycombinator.com/item?id=40465090.
      - ribit 2y ago
        > I agree with you we do not only want "very wide SIMD", and it seems to me that 2x512-bit (Intel) or 4x256 (AMD) are actually a good middle ground. I'd already classify this as "very wide". And the story is far from being that simple. Intel's 512-bit implementation is very area- and power-hungry, so much so that Intel is dropping the 512-bit SIMD altogether. AMD has 4x add units, but only two are capable of multiplication. So if your code mostly does FP addition, you get good performance. If your workflows are more complex, not so much. The thing is that on many real-world SIMD workloads, Apple's 4x128bit either matches or outperforms either Intel's or AMD's implementation. And that on a core that runs lower clock and has less L1D bandwidth. Flexibility and symmetric ALU capabilities seems to be the king here. > Sure, it's https://news.ycombinator.com/item?id=40465090 https://news.ycombinator.com/item?id=40465090 Ah, that is what you meant. Thank you for linking the post! My comment would be that this is not about 128b or 256b SIMD per se but about implementation details. There is nothing stopping ARM from designing a core with more mask write ports. Apparently, they felt this was not worth the cost. Other vendors might feel differently. I'd say this is similar to AMD shipping only two FMA units instead of four. Other vendors might feel differently.

by hajile 2y ago

dzaima 2y ago

You can't do 2048 bits of addition in one SVE instruction; not portably, at least (and definitely not on any existing hardware). While the maximum SVE register size is 2048 bits, the minimum is 128 bits, and the hardware chooses the supported register size, not the programmer. For portable SVE, your code needs to work for all of those widths, not just the smallest or largest. (of related note is RISC-V RVV, which allows you to group up to 8 registers together, allowing a minimum portable operation width of 128×8 = 1024 bits in a single instruction (and up to 65536×8 = 64KB for hypothetical crazy hardware with max VLEN), but SVE/SVE2 don't have any equivalent)

brigade 2y ago

A for() loop does the same thing at the cost of like 3 instructions. 4x128b has the flexibility that you don't need 512b wide operations on the same data to keep the ALUs fed. If you have 512b wide operations being split to 4x128b instructions, great, otherwise the massive OoOE window of modern chips can decode the next few loop iterations to keep the ALUs fed, or even pull instructions from a completely different kernel.

ribit 2y ago

I don't see how this would work out beneficially. Let's say your hardware can join 4x128b units as a virtual 512-bit SVE SIMD unit. This means you have to advertise VL as 512bit for reasons of consistency. Yes, you will save some entries in the reorder buffer if you encounter a single SVE instruction, but if the code contains independent SVE streams, you will be stalled. Moreso, not all operations will utilize all 512 register bits, so your occupancy might suffer. The only scenario I see this feature working out is if you are decode or reorder buffer limited. Neither is a problem for modern high-performance ARM cores. With x86, it might be a different story. From what I understand, AVX512 instructions can be quite large. Modern out-of-order cores are already good at superscalar execution, so why not let them do their job? 4x128b units give you much more flexibility and better execution granularity.

janwas 2y ago
On x86 at least, the cost of OoO is astonishing - more pJ per instruction dispatch than the operation itself. Amortizing that over more operations is the whole point of SIMD. I have not yet seen such data for Arm. That aside, see the "cmp" sibling thread for a major (4x penalty) downside to 4x128.
- ribit 2y ago
  Yes, OoO is expensive — after all, that is the cost of performance. Very wide SIMD is great for energy efficiency if that is what your compute patterns require (there is a good reason why GPUs are in-order very wide SMT SIMD processors). Is this the best choice for a general-purpose CPU? That I am not so sure about. A CPU needs to be able to run all kinds of code. A single wide SIMD unit is great for some problems, but it won't deliver good performance if you need more flexibility. Could you point me to the "cmp" thread you mentioned? I don't know where to look for it.
  - janwas 2y ago
    I agree with you we do not only want "very wide SIMD", and it seems to me that 2x512-bit (Intel) or 4x256 (AMD) are actually a good middle ground. Sure, it's https://news.ycombinator.com/item?id=40465090 https://news.ycombinator.com/item?id=40465090.
    - ribit 2y ago
      > I agree with you we do not only want "very wide SIMD", and it seems to me that 2x512-bit (Intel) or 4x256 (AMD) are actually a good middle ground. I'd already classify this as "very wide". And the story is far from being that simple. Intel's 512-bit implementation is very area- and power-hungry, so much so that Intel is dropping the 512-bit SIMD altogether. AMD has 4x add units, but only two are capable of multiplication. So if your code mostly does FP addition, you get good performance. If your workflows are more complex, not so much. The thing is that on many real-world SIMD workloads, Apple's 4x128bit either matches or outperforms either Intel's or AMD's implementation. And that on a core that runs lower clock and has less L1D bandwidth. Flexibility and symmetric ALU capabilities seems to be the king here. > Sure, it's https://news.ycombinator.com/item?id=40465090 https://news.ycombinator.com/item?id=40465090 Ah, that is what you meant. Thank you for linking the post! My comment would be that this is not about 128b or 256b SIMD per se but about implementation details. There is nothing stopping ARM from designing a core with more mask write ports. Apparently, they felt this was not worth the cost. Other vendors might feel differently. I'd say this is similar to AMD shipping only two FMA units instead of four. Other vendors might feel differently.