This package modifies the AES-GCM implementation from Go's standard library to use 12 byte tag sizes. It is not intended for a general audience, and used in quic-go.
To make use of the in-place encryption / decryption feature, the dst parameter to Seal and Open should be 16 bytes longer than plaintext, not 12.
Command for testing:
go test . --bench=. && GOARCH=386 go test . --bench=.
The output (on my machine):
BenchmarkAESGCMSeal1K-8 3000000 467 ns/op 2192.37 MB/s
BenchmarkAESGCMOpen1K-8 3000000 416 ns/op 2456.72 MB/s
BenchmarkAESGCMSeal8K-8 500000 2742 ns/op 2986.53 MB/s
BenchmarkAESGCMOpen8K-8 500000 2791 ns/op 2934.65 MB/s
PASS
ok github.com/lucas-clemente/aes12 6.383s
BenchmarkAESGCMSeal1K-8 50000 35233 ns/op 29.06 MB/s
BenchmarkAESGCMOpen1K-8 50000 34529 ns/op 29.66 MB/s
BenchmarkAESGCMSeal8K-8 5000 262678 ns/op 31.19 MB/s
BenchmarkAESGCMOpen8K-8 5000 267296 ns/op 30.65 MB/s
PASS
ok github.com/lucas-clemente/aes12 6.972s