go-performance

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name: go-performance description: Use when optimizing Go code, investigating slow performance, or writing performance-critical sections. Also use when a user mentions slow Go code, string concatenation in loops, or asks about benchmarking, even if the user doesn't explicitly mention performance patterns. Does not cover concurrent performance patterns (see go-concurrency). allowed-tools: Bash(bash:*)

Go Performance Patterns

Resource Routing

• scripts/bench-compare.sh - Run when comparing benchmark results, saving baselines, or producing JSON benchmark metadata.
• references/BENCHMARKS.md - Read when writing benchmarks, using benchstat, or profiling with pprof.
• references/STRING-OPTIMIZATION.md - Read when optimizing string conversion, concatenation, or byte/string boundaries.

Performance-specific guidelines apply only to the hot path. Don't prematurely optimize—focus these patterns where they matter most.

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Prefer strconv over fmt

When converting primitives to/from strings, strconv is faster than fmt:

s := strconv.Itoa(rand.Int()) // ~2x faster than fmt.Sprint()

Approach	Speed	Allocations
fmt.Sprint	143 ns/op	2 allocs/op
strconv.Itoa	64.2 ns/op	1 allocs/op

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Avoid Repeated String-to-Byte Conversions

Convert a fixed string to []byte once outside the loop:

data := []byte("Hello world")
for b.Loop() { // Go 1.24+; use b.N loops only for older Go
    w.Write(data) // ~7x faster than []byte("...") each iteration
}

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Prefer Specifying Container Capacity

Specify container capacity where possible to allocate memory up front. This minimizes subsequent allocations from copying and resizing as elements are added.

Map Capacity Hints

Provide capacity hints when initializing maps with make():

m := make(map[string]os.DirEntry, len(files))

Note: Unlike slices, map capacity hints do not guarantee complete preemptive allocation—they approximate the number of hashmap buckets required.

Slice Capacity

Provide capacity hints when initializing slices with make(), particularly when appending:

data := make([]int, 0, size)

Unlike maps, slice capacity is not a hint—the compiler allocates exactly that much memory. Subsequent append() operations incur zero allocations until capacity is reached.

Approach	Time (100M iterations)
No capacity	2.48s
With capacity	0.21s

The capacity version is ~12x faster due to zero reallocations during append.

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Pass Values

Don't pass pointers as function arguments just to save a few bytes. If a function refers to its argument x only as *x throughout, then the argument shouldn't be a pointer.

func process(s string) { // not *string — strings are small fixed-size headers
    fmt.Println(s)
}

Common pass-by-value types: string, io.Reader, small structs.

Exceptions:

• Large structs where copying is expensive
• Small structs that might grow in the future

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String Concatenation

Choose the right strategy based on complexity:

Method	Best For
+	Few strings, simple concat
fmt.Sprintf	Formatted output with mixed types
strings.Builder	Loop/piecemeal construction
strings.Join	Joining a slice
Backtick literal	Constant multi-line text

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Benchmarking and Profiling

Always measure before and after optimizing. Use Go's built-in benchmark framework and profiling tools.

go test -bench=. -benchmem -count=10 ./...

Validation: After applying optimizations, run bash scripts/bench-compare.sh to measure the actual impact. Only keep optimizations with measurable improvement.

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Quick Reference

Pattern	Bad	Good	Improvement
Int to string	fmt.Sprint(n)	strconv.Itoa(n)	~2x faster
Repeated []byte	[]byte("str") in loop	Convert once outside	~7x faster
Map initialization	make(map[K]V)	make(map[K]V, size)	Fewer allocs
Slice initialization	make([]T, 0)	make([]T, 0, cap)	~12x faster
Small fixed-size args	*string, *io.Reader	string, io.Reader	No indirection
Simple string join	s1 + " " + s2	(already good)	Use + for few strings
Loop string build	Repeated +=	strings.Builder	O(n) vs O(n²)

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Related Skills

• Data structures: See go-data-structures when choosing between slices, maps, and arrays, or understanding allocation semantics
• Declaration patterns: See go-declarations when using make with capacity hints or initializing maps and slices
• Concurrency: See go-concurrency when parallelizing work across goroutines or using sync.Pool for buffer reuse
• Style principles: See go-style-core when deciding whether an optimization is worth the readability cost