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Chandler Carruth has a good talk with an example of why speculative execution is critical for performance. https://www.youtube.com/watch?v=2EWejmkKlxs https://w

by pkaye 8y ago

Chandler Carruth has a good talk with an example of why speculative execution is critical for performance. https://www.youtube.com/watch?v=2EWejmkKlxs https://www.youtube.com/watch?v=2EWejmkKlxs It starts around 36m13s time frame.

dnautics 8y ago
conceivably this could be all put into the compiler.
- pkaye 8y ago
  You mean speculative execution? Do you know of an sample implementation?
- twtw 8y ago
  Conceivable, yes. Practical? Not so much. This is an idea that I personally love, but that hasn't fared well so far. Compilers are not as good as assigning instruction schedules statically as hardware can do dynamically.
  - dnautics 8y ago
    curious as to why hardware can do it dynamically while software can't. It's all logic in the end. I can understand "not being able to statically compile it because every architecture is different" but, presuming our compiler compiled to a specific platform - why wouldn't it be able to dynamically rearrange in, say, a JITted fashion using exactly whatever logic is available in the hardware.
    - twtw 8y ago
      Hopefully I'll put together a more technical answer in a while, but for now I'll just point out that when talking about performance, reducing things to "it's all logic in the end" makes little sense. We could emulate a modern CPU on an 8-bit micro controller, but the performance would be bad.
ken 8y ago
That's a fascinating video about how modern processors work, but I don't see here why it's critical for performance. If you built a CPU without speculation, how bad would perf be? What other features could you still use? How much do common algorithms depend on speculation?
- pkaye 8y ago
  Superscalar processors have a deep pipeline with many execution units and keep a lot of instructions in flight so a penalty of a misprediction or stall is significant. Every time it reaches a branch instruction that depends on a result which is not yet available it would need to speculate or stall. Most programs consist of small amounts of compute code followed by a branch that might depends on the results of that code.

by pkaye 8y ago

dnautics 8y ago

conceivably this could be all put into the compiler.

pkaye 8y ago
You mean speculative execution? Do you know of an sample implementation?
twtw 8y ago
Conceivable, yes. Practical? Not so much. This is an idea that I personally love, but that hasn't fared well so far. Compilers are not as good as assigning instruction schedules statically as hardware can do dynamically.
- dnautics 8y ago
  curious as to why hardware can do it dynamically while software can't. It's all logic in the end. I can understand "not being able to statically compile it because every architecture is different" but, presuming our compiler compiled to a specific platform - why wouldn't it be able to dynamically rearrange in, say, a JITted fashion using exactly whatever logic is available in the hardware.
  - twtw 8y ago
    Hopefully I'll put together a more technical answer in a while, but for now I'll just point out that when talking about performance, reducing things to "it's all logic in the end" makes little sense. We could emulate a modern CPU on an 8-bit micro controller, but the performance would be bad.

ken 8y ago

That's a fascinating video about how modern processors work, but I don't see here why it's critical for performance. If you built a CPU without speculation, how bad would perf be? What other features could you still use? How much do common algorithms depend on speculation?

pkaye 8y ago
Superscalar processors have a deep pipeline with many execution units and keep a lot of instructions in flight so a penalty of a misprediction or stall is significant. Every time it reaches a branch instruction that depends on a result which is not yet available it would need to speculate or stall. Most programs consist of small amounts of compute code followed by a branch that might depends on the results of that code.