Array vs. Slice in Go: Why is Local Slice Element Access Faster?

Array vs Slice: Speed Comparison of Element Access

In Go, arrays and slices are commonly used data structures for storing an ordered collection of elements. One notable difference between the two is their performance characteristics.

When it comes to accessing elements, accessing elements in an array is expected to be faster than accessing elements in a slice. This is because a slice represents a view of an underlying array, and accessing its elements may involve an additional level of indirection.

However, recent benchmarks have shown surprising results. When comparing local arrays and slices, the results indicate that local slices are significantly faster for accessing elements than local arrays.

Analysis of the Results

Upon examining the assembly code for both local arrays and local slices, a key difference becomes apparent. The array version repeatedly loads the address of the array (a) from memory for each array access.

LEAQ    "".a+1000(SP),BX

In contrast, the slice version computes exclusively on registers after initially loading the slice data from memory.

LEAQ    (DX)(SI*1),BX

This difference in memory access patterns likely accounts for the performance advantage of local slices. By avoiding the repeated memory loads to access the array's base address, slices benefit from faster element access.

Additionally, the array version calls into the runtime.duffcopy routine, while the slice version does not. Duffcopy is an assembly routine optimized for bulk memory copying, and its usage in the array version may further contribute to its slower performance.

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