File sizes

While the impact to build times is negligible, this page investigates how generics affect the file sizes of compiled code:

The example

This page describes how to run BenchmarkGoBuild, a Go-based benchmark that inspects the file sizes of:

  • a package archive
  • a binary executable

Each build varies based on the:

  • list type, ex. boxed, generic, typed
  • number of distinct type definitions

The packages used by this benchmark are:

  • ./lists/boxed: defines type List []interface{}
  • ./lists/typed: defines zero to many list types based on build tags
  • ./lists/generic: defines type List[T any] []T

The typed and generic packages are also subject to the following build tags:

  • int: activates that package’s list of int
  • int8: activates that package’s list of int8
  • int16: activates that package’s list of int16
  • int32: activates that package’s list of int32
  • int64: activates that package’s list of int64

The benchmark

Run the benchmark with the following command:

  1. docker run -it --rm go-generics-the-hard-way \
  2.   go test -bench GoBuild -run GoBuild -count 1 -v ./06-benchmarks

The following table was generated by piping the above command to hack/b2md.py -t filesize:

Artifact type Number of types File size (bytes) - typed File size (bytes) - generic Increase (bytes) Increase (%)
pkg 0 818 1194 376 45.97
1 5948 11534 5586 93.91
2 10458 20214 9756 93.29
3 15004 28932 13928 92.83
4 19552 37648 18096 92.55
5 24098 46278 22180 92.04
bin 0 1121184 1121184 0 0
1 1121184 1125280 4096 0.37
2 1125280 1125280 0 0
3 1125280 1125280 0 0
4 1125280 1125280 0 0
5 1125280 1125280 0 0

Validating the artifacts

To validate each of the produced artifacts contain the expected types, use the following commands:

  1. # Print types in each generic package archive
  2. find -s ./06-benchmarks -name "generic-*-types.a" -type f -print0 | \
  3. xargs -0 -I% sh -c "echo % && go tool objdump -S % | grep -o 'go.shape.int[[:digit:]_]\{1,\}' | sort -ru && echo"
  1. # Print types in each generic binary executable
  2. find -s ./06-benchmarks -name "generic-*-types.bin" -type f -print0 | \
  3. xargs -0 -I% sh -c "echo % && go tool objdump -S % | grep -o 'int[[:digit:]]\{0,\}List' | sort -ru && echo"
  1. # Print types in each typed package archive
  2. find -s ./06-benchmarks -name "typed-*-types.a" -type f -print0 | \
  3. xargs -0 -I% sh -c "echo % && go tool objdump -S % | grep -o 'Int[[:digit:]]\{0,\}List' | sort -ru && echo"
  1. # Print types in each typed binary executable
  2. find -s ./06-benchmarks -name "typed-*-types.bin" -type f -print0 | \
  3. xargs -0 -I% sh -c "echo % && go tool objdump -S % | grep -o 'int[[:digit:]]\{0,\}List' | sort -ru && echo"

In all cases the output should indicate a file with:

  • 0 types has no match
  • 1 type matches int
  • 2 types match int and int8
  • 3 types match int, int8, and int16
  • 4 types match int, int8, int16, and int32
  • 5 types match int, int8, int16, int32, and int64

Key takeaways

  • It appears that package archives built from generic code are consistently close to twice as large versus as non-generic counterparts (golang/go#50438)
  • At the same time, generic types appear to have no discernable impact on the size of compiled, binary executables

Next: Lessons learned