---

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). license: Apache-2.0 metadata: sources: "Uber Style Guide, Google Style Guide, Go Wiki CodeReviewComments" allowed-tools: Bash(bash:*)

Go Performance Patterns

Available Scripts

• scripts/bench-compare.sh — Runs Go benchmarks N times with optional baseline comparison via benchstat. Supports saving results for future comparison. Run bash scripts/bench-compare.sh --help for options.

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 |

Read references/STRING-OPTIMIZATION.md when choosing between strconv and fmt for type conversions, or for the full conversion table.

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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 i := 0; i < b.N; i++ {
    w.Write(data) // ~7x faster than []byte("...") each iteration
}

Read references/STRING-OPTIMIZATION.md when optimizing repeated byte conversions in hot loops.

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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 |

Read references/STRING-OPTIMIZATION.md when choosing a string concatenation strategy, using strings.Builder in loops, or deciding between fmt.Sprintf and manual concatenation.

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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 ./...

Read references/BENCHMARKS.md when writing benchmarks, comparing results with benchstat, profiling with pprof, or interpreting benchmark output.

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