3 ms·

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 th

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__ 5y 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 5y ago
  The Itantium did care about memory its one of the reasons they had massive caches compared to other archs of the era.
twoodfin 5y 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__ 5y 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.
  - saagarjha 5y ago
    https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/AArch64/AArch64.td https://github.com/llvm/llvm-project/blob/main/llvm/lib/Targ... is probably a good start for LLVM.
    - mhh__ 5y ago
      I meant the actual scheduling algorithm - from what I can tell GCC seems to basically use an SM based in order scheduler with the aim of not stalling the decoder. Currently, I'm mostly interested in basic block scheduling rather than trace scheduling or anything of that order.

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__ 5y 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 5y ago
The Itantium did care about memory its one of the reasons they had massive caches compared to other archs of the era.

twoodfin 5y 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__ 5y 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.
- saagarjha 5y ago
  https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/AArch64/AArch64.td https://github.com/llvm/llvm-project/blob/main/llvm/lib/Targ... is probably a good start for LLVM.
  - mhh__ 5y ago
    I meant the actual scheduling algorithm - from what I can tell GCC seems to basically use an SM based in order scheduler with the aim of not stalling the decoder. Currently, I'm mostly interested in basic block scheduling rather than trace scheduling or anything of that order.