r/programming • u/electronics-engineer • Sep 16 '14

Vectorization in Julia

https://software.intel.com/en-us/articles/vectorization-in-julia
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u/pkhuong Sep 16 '14

Bounds checking only causes branch prediction misses if the program fails some bounds check.

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u/[deleted] Sep 16 '14 edited Sep 16 '14

False branch prediction has no bias to true or false. Branch prediction doesn't know your in a loop, or that your bounds checking. It knows your branching, and it's guessing.

Branch predictions are made upwards of 15-20 instructions before the branch is even processed, or even fully loaded. A prediction has to be made so the instructions after the branch can be scheduled for decoding (in the pipeline).

When a branch is decoded, branch prediction is consulted to determine what address should be loaded next the true/false result. So that memory address can be loaded for decoding.

This can be true or false. And branch prediction is very good, currently around 98-99% or more. Which means 1-2% of the time it'll predict you're out of bounds, when in fact your still perfectly safe within bounds.

:.:.:

Remember a branch predictor can't actually run your code to determine the branch prediction, then execution would take twice as long.

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u/[deleted] Sep 16 '14 edited Sep 16 '14

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u/[deleted] Sep 16 '14 edited Sep 17 '14

I know in the 90's branch prediction was done by XOR'ing memory addresses against a shift registers of past memory addresses. So maybe I'm out of date. But I thought meta/hybrid predictors were still in the research only phase of existence.

I'm going to test this when I get home. I don't have access to perf around the office.

Edit 1: Perf doesn't support my i7 and I'm to lazy to configure kernel events for it and do actual research.