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The fact that Intel and AMD apparently don't prioritize integer division could suggest that their profiling suggests it's not worth it, but with Apple's transis

by mhh__ 6y ago

The fact that Intel and AMD apparently don't prioritize integer division could suggest that their profiling suggests it's not worth it, but with Apple's transistor budget at the moment they can afford it. Also keep in mind that this Xeon might not be really made for number crunching (not really sure)?

masklinn 6y ago
> The fact that Intel and AMD apparently don't prioritize integer division could suggest that their profiling suggests it's not worth it, but with Apple's transistor budget at the moment they can afford it. An other possibility is that Apple has a very different profiling base e.g. iOS applications, whereas Intel and AMD would have more artificial workloads, or be bound by workloads / profiles from scientific computing or the like (video games)?
criddell 6y ago
Would it be fair to characterize the M1 as being made for number crunching?
- mhh__ 6y ago
  Refining number crunching to mean single threaded performance I would say yes, or at least definitely more so than the Intel chip
- gameswithgo 6y ago
  Not any more than an intel/amd/etc cpu is. Like that XEON cpu is gonna crunch more numbers, just due to more cores.
  - mhh__ 6y ago
    If it was intended to be used in the cloud for example it's going to be doing more work but probably designed around a memory-bound load rather than integer throughput.
pbsd 6y ago
Intel greatly improved their divider implementation between Skylake and Icelake. The measurements in the OP are on Skylake-SP, prior to these improvements.
rodgerd 6y ago
Apple's chip designers have the advantage, I assume, of being able to wander down a hallway and ask what the telemetry from iOS and MacOS devices are telling them about real-world use.
- mhh__ 6y ago
  Most of Intel's volume is probably shipped to customers who either don't care or buy a lot of CPUs in one go, so the advantage of this probably isn't quite as apparent as you'd imagine. What can definitely play a role is that (I don't think it's as much of a problem these days, but it definitely has been in the past) is the standard "benchmark" suites that chipmakers can beat each other over the head with e.g. I think it was Itanium that had a bunch of integer functional units mainly for the purpose of getting better SPEC numbers rather than working on the things that actually make programs fast (MEMORY) - I was maybe 1 or 2 when this chip came out, so this is nth-hand gossip, however.
  - anfilt 6y ago
    The Itantium did care about memory its one of the reasons they had massive caches compared to other archs of the era.
- twoodfin 6y ago
  They also have the advantage of controlling the compiler used by effectively the entire development ecosystem. And have groomed that ecosystem through carrots and sticks to upgrade their compilers and rebuild their applications regularly (or use bitcode!) A compiler code generator that knows about a hypothetical two divide units (or just a much more efficient single unit) could be much more effective statically scheduling around them. I’d guess that the bulk of the software running on the highest margin Intel Xeons was compiled some years ago and tuned for microarchitectures even older.
  - mhh__ 6y ago
    >A compiler code generator that knows about a hypothetical two divide units (or just a much more efficient single unit) could be much more effective statically scheduling around them. I'm still completely blind to how they are actually used but GCC and LLVM both have pretty good internal representations of the microarchitecture they are compiling for. If I ever work it out I'll write a blog post about it, but this is an area where GCC and LLVM are both equally impenetrable.

by mhh__ 6y ago

masklinn 6y ago

> The fact that Intel and AMD apparently don't prioritize integer division could suggest that their profiling suggests it's not worth it, but with Apple's transistor budget at the moment they can afford it. An other possibility is that Apple has a very different profiling base e.g. iOS applications, whereas Intel and AMD would have more artificial workloads, or be bound by workloads / profiles from scientific computing or the like (video games)?

criddell 6y ago

Would it be fair to characterize the M1 as being made for number crunching?

mhh__ 6y ago
Refining number crunching to mean single threaded performance I would say yes, or at least definitely more so than the Intel chip
gameswithgo 6y ago
Not any more than an intel/amd/etc cpu is. Like that XEON cpu is gonna crunch more numbers, just due to more cores.
- mhh__ 6y ago
  If it was intended to be used in the cloud for example it's going to be doing more work but probably designed around a memory-bound load rather than integer throughput.

pbsd 6y ago

Intel greatly improved their divider implementation between Skylake and Icelake. The measurements in the OP are on Skylake-SP, prior to these improvements.

rodgerd 6y ago

Apple's chip designers have the advantage, I assume, of being able to wander down a hallway and ask what the telemetry from iOS and MacOS devices are telling them about real-world use.

mhh__ 6y ago
Most of Intel's volume is probably shipped to customers who either don't care or buy a lot of CPUs in one go, so the advantage of this probably isn't quite as apparent as you'd imagine. What can definitely play a role is that (I don't think it's as much of a problem these days, but it definitely has been in the past) is the standard "benchmark" suites that chipmakers can beat each other over the head with e.g. I think it was Itanium that had a bunch of integer functional units mainly for the purpose of getting better SPEC numbers rather than working on the things that actually make programs fast (MEMORY) - I was maybe 1 or 2 when this chip came out, so this is nth-hand gossip, however.
- anfilt 6y ago
  The Itantium did care about memory its one of the reasons they had massive caches compared to other archs of the era.
twoodfin 6y ago
They also have the advantage of controlling the compiler used by effectively the entire development ecosystem. And have groomed that ecosystem through carrots and sticks to upgrade their compilers and rebuild their applications regularly (or use bitcode!) A compiler code generator that knows about a hypothetical two divide units (or just a much more efficient single unit) could be much more effective statically scheduling around them. I’d guess that the bulk of the software running on the highest margin Intel Xeons was compiled some years ago and tuned for microarchitectures even older.
- mhh__ 6y ago
  >A compiler code generator that knows about a hypothetical two divide units (or just a much more efficient single unit) could be much more effective statically scheduling around them. I'm still completely blind to how they are actually used but GCC and LLVM both have pretty good internal representations of the microarchitecture they are compiling for. If I ever work it out I'll write a blog post about it, but this is an area where GCC and LLVM are both equally impenetrable.