11b2853e6f
The previous behavior directly contradicted the docs that have been in place for years: To unmarshal a JSON array into a slice, Unmarshal resets the slice length to zero and then appends each element to the slice. We could use reflect.New to create a new element and reflect.Append to then append it to the destination slice, but benchmarks have shown that reflect.Append is very slow compared to the code that manually grows a slice in this file. Instead, if we're decoding into an element that came from the original backing array, zero it before decoding into it. We're going to be using the CodeDecoder benchmark, as it has a slice of struct pointers that's decoded very often. Note that we still reuse existing values from arrays being decoded into, as the documentation agrees with the existing implementation in that case: To unmarshal a JSON array into a Go array, Unmarshal decodes JSON array elements into corresponding Go array elements. The numbers with the benchmark as-is might seem catastrophic, but that's only because the benchmark is decoding into the same variable over and over again. Since the old decoder was happy to reuse slice elements, it would save a lot of allocations by not having to zero and re-allocate said elements: name old time/op new time/op delta CodeDecoder-8 10.4ms ± 1% 10.9ms ± 1% +4.41% (p=0.000 n=10+10) name old speed new speed delta CodeDecoder-8 186MB/s ± 1% 178MB/s ± 1% -4.23% (p=0.000 n=10+10) name old alloc/op new alloc/op delta CodeDecoder-8 2.19MB ± 0% 3.59MB ± 0% +64.09% (p=0.000 n=10+10) name old allocs/op new allocs/op delta CodeDecoder-8 76.8k ± 0% 92.7k ± 0% +20.71% (p=0.000 n=10+10) We can prove this by moving 'var r codeResponse' into the loop, so that the benchmark no longer reuses the destination pointer. And sure enough, we no longer see the slow-down caused by the extra allocations: name old time/op new time/op delta CodeDecoder-8 10.9ms ± 0% 10.9ms ± 1% -0.37% (p=0.043 n=10+10) name old speed new speed delta CodeDecoder-8 177MB/s ± 0% 178MB/s ± 1% +0.37% (p=0.041 n=10+10) name old alloc/op new alloc/op delta CodeDecoder-8 3.59MB ± 0% 3.59MB ± 0% ~ (p=0.780 n=10+10) name old allocs/op new allocs/op delta CodeDecoder-8 92.7k ± 0% 92.7k ± 0% ~ (all equal) I believe that it's useful to leave the benchmarks as they are now, because the decoder does reuse memory in some cases. For example, existing map elements are reused. However, subtle changes like this one need to be benchmarked carefully. Finally, add a couple of tests involving both a slice and an array of structs. Fixes #21092. Change-Id: I8b1194f25e723a31abd146fbfe9428ac10c1389d Reviewed-on: https://go-review.googlesource.com/c/go/+/191783 Reviewed-by: Ian Lance Taylor <iant@golang.org> Run-TryBot: Ian Lance Taylor <iant@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> |
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