How does Go optimize string comparison for literal strings?

Exploring String Comparison Internals in Go

Go's string comparison mechanism is notable for its conciseness, as it does not require any specific functions. While this may raise questions about its efficiency, we dive into the runtime operations behind this comparison process.

According to the documentation at http://golang.org/ref/spec#Comparison_operators, Go aligns its string comparison with its spec, implementing an O(n) comparison based on the length of the strings. However, to optimize performance, Go has tailored its approach for literal strings.

When comparing literal strings, Go employs a two-step mechanism:

1. Shortcut Check: The runtime first conducts a quick check to ascertain if both operands reside in the same in-memory string. If they do, the comparison is concluded as "true."
2. runtime.eqstring: If the shortcut check fails, the runtime delegates the comparison to the runtime.eqstring function. This function compares the strings byte by byte and returns a boolean result.

An assembly dump provides further insight into this process:

--- prog list "main" ---
17 (foo.go:6) CALL    ,runtime.eqstring+0(SB)

Line 17 demonstrates the invocation of runtime.eqstring when the simple check fails.

In conclusion, string comparison in Go involves a nuanced approach that leverages a runtime function for literal strings and a rudimentary byte-by-byte comparison for other scenarios. While this implementation prioritizes efficiency for common cases, it maintains the O(n) performance ceiling for all string comparisons.

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