Encode structs directly to output buffer.

[benluddy]

Description

For variable-length structs (structs with omitempty fields), encoding to the unused capacity at the
end of the output buffer while counting nonempty items is cheaper than using a separate temporary
buffer (no pool interactions and better spatial locality). Copying the items can be avoided entirely
by reserving space in the output buffer for the head if the encoded length of the head can be
predicted before checking optional fields.

                                                                     │ before.txt  │              after.txt              │
                                                                     │   sec/op    │   sec/op     vs base                │
Marshal/Go_struct_to_CBOR_map                                          1.404µ ± 0%   1.408µ ± 1%        ~ (p=0.170 n=10)
Marshal/Go_struct_many_fields_all_omitempty_all_empty_to_CBOR_map      443.8n ± 0%   430.6n ± 0%   -2.99% (p=0.000 n=10)
Marshal/Go_struct_some_fields_all_omitempty_all_empty_to_CBOR_map      181.7n ± 0%   163.5n ± 0%  -10.04% (p=0.000 n=10)
Marshal/Go_struct_many_fields_all_omitempty_all_nonempty_to_CBOR_map   813.5n ± 0%   784.8n ± 0%   -3.53% (p=0.000 n=10)
Marshal/Go_struct_some_fields_all_omitempty_all_nonempty_to_CBOR_map   300.8n ± 0%   275.4n ± 0%   -8.43% (p=0.000 n=10)
Marshal/Go_struct_many_fields_one_omitempty_to_CBOR_map                763.8n ± 0%   727.7n ± 0%   -4.73% (p=0.000 n=10)
Marshal/Go_struct_some_fields_one_omitempty_to_CBOR_map                284.2n ± 0%   257.6n ± 0%   -9.36% (p=0.000 n=10)
Marshal/Go_struct_keyasint_to_CBOR_map                                 1.422µ ± 0%   1.414µ ± 1%   -0.56% (p=0.029 n=10)
Marshal/Go_struct_toarray_to_CBOR_array                                1.341µ ± 1%   1.338µ ± 1%        ~ (p=0.340 n=10)
MarshalCanonical/Go_struct_to_CBOR_map                                 386.4n ± 0%   392.4n ± 0%   +1.57% (p=0.000 n=10)
MarshalCanonical/Go_struct_to_CBOR_map_canonical                       386.9n ± 0%   384.8n ± 0%   -0.52% (p=0.001 n=10)
geomean                                                                560.5n        540.4n        -3.59%

                                                                     │ before.txt │              after.txt              │
                                                                     │    B/op    │    B/op     vs base                 │
Marshal/Go_struct_to_CBOR_map                                          208.0 ± 0%   208.0 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_many_fields_all_omitempty_all_empty_to_CBOR_map      1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_some_fields_all_omitempty_all_empty_to_CBOR_map      1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_many_fields_all_omitempty_all_nonempty_to_CBOR_map   176.0 ± 0%   176.0 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_some_fields_all_omitempty_all_nonempty_to_CBOR_map   48.00 ± 0%   48.00 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_many_fields_one_omitempty_to_CBOR_map                160.0 ± 0%   160.0 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_some_fields_one_omitempty_to_CBOR_map                48.00 ± 0%   48.00 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_keyasint_to_CBOR_map                                 192.0 ± 0%   192.0 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_toarray_to_CBOR_array                                192.0 ± 0%   192.0 ± 0%       ~ (p=1.000 n=10) ¹
MarshalCanonical/Go_struct_to_CBOR_map                                 64.00 ± 0%   64.00 ± 0%       ~ (p=1.000 n=10) ¹
MarshalCanonical/Go_struct_to_CBOR_map_canonical                       64.00 ± 0%   64.00 ± 0%       ~ (p=1.000 n=10) ¹
geomean                                                                46.18        46.18       +0.00%
¹ all samples are equal

                                                                     │ before.txt │              after.txt              │
                                                                     │ allocs/op  │ allocs/op   vs base                 │
Marshal/Go_struct_to_CBOR_map                                          1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_many_fields_all_omitempty_all_empty_to_CBOR_map      1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_some_fields_all_omitempty_all_empty_to_CBOR_map      1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_many_fields_all_omitempty_all_nonempty_to_CBOR_map   1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_some_fields_all_omitempty_all_nonempty_to_CBOR_map   1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_many_fields_one_omitempty_to_CBOR_map                1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_some_fields_one_omitempty_to_CBOR_map                1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_keyasint_to_CBOR_map                                 1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
Marshal/Go_struct_toarray_to_CBOR_array                                1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
MarshalCanonical/Go_struct_to_CBOR_map                                 1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
MarshalCanonical/Go_struct_to_CBOR_map_canonical                       1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
geomean                                                                1.000        1.000       +0.00%
¹ all samples are equal

PR Was Proposed and Welcomed in Currently Open Issue

• This PR was proposed and welcomed by maintainer(s) in issue #___
• Closes or Updates Issue #___

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[fxamacker]

@benluddy Thanks for opening this PR!

In addition to getting non-empty field count in the first pass, what do you think about also getting non-empty fields ([]*reflect.Value) as well in the same pass? So we don't need to perform same operations in both passes.

For example:

• in the first pass, get non-empty field count kvcount, also create and populate a non-empty field reflect values fvs []*reflect.Value on stack.
• in the second pass, encode non-empty field if fvs[i] != nil.

Benchmarks show more improvement when updated with these changes.

Thoughts?

[benluddy]

Instead of making two passes, it now encodes the items to the output buffer while counting, encodes the head at the end, and uses excess capacity in the output buffer to swap the positions of the encoded head and the encoded items. This turned out to be faster. Then I realized that you'll usually have a variable-length struct whose head encodes to the same number of bytes regardless of the number of items (e.g. any struct with fewer than 24 fields). In that case, it reserves space in the output buffer for the head, encodes the items, then overwrites the head bytes in the output buffer at the end, once it knows the actual map size.

[fxamacker]

@benluddy Thanks for updating this PR! The one pass approach sounds good! 👍

I have some suggestions to simplify the code for your consideration:

• have a utility function to return length of encoded map head with element count
• only cache maxHeadLen in encodingStructType
• in encodeStruct(), we can:
  • reserve bytes of maxHeadLen before encoding elements
  • overwrite the reserved bytes with the real head after elements are encoded
  • if real head len < max head len, shift encoded elements to the left and truncate underlying buffer

Thoughts?

[benluddy]

@benluddy Thanks for updating this PR! The one pass approach sounds good! 👍

I have some suggestions to simplify the code for your consideration:

* have a utility function to return length of encoded map head with element count

* only cache `maxHeadLen` in `encodingStructType`

* in `encodeStruct()`, we can:
  
  * reserve bytes of `maxHeadLen` before encoding elements
  * overwrite the reserved bytes with the real head after elements are encoded
  * if real head len < max head len, shift encoded elements to the left and truncate underlying buffer

Thoughts?

Absolutely! For some reason I was afraid of overlapping copies, but they are clearly safe according to the spec (https://go.dev/ref/spec#Appending_and_copying_slices). I'll implement your suggestions and rerun the benchmarks. Thanks!

[benluddy]

@fxamacker I just pushed those changes. Benchmarks looks good! As expected, it's a bit faster on the interesting cases by avoiding the extra copies, and the worst-case scratch buffer space needed is only a few bytes instead of being proportional to the encoded size of the entire map:

                                                                     │  prev.txt   │              next.txt              │
                                                                     │   sec/op    │   sec/op     vs base               │
Marshal/Go_struct_to_CBOR_map                                          1.403µ ± 1%   1.408µ ± 1%       ~ (p=0.672 n=10)
Marshal/Go_struct_many_fields_all_omitempty_all_empty_to_CBOR_map      425.5n ± 0%   430.6n ± 0%  +1.20% (p=0.000 n=10)
Marshal/Go_struct_some_fields_all_omitempty_all_empty_to_CBOR_map      163.5n ± 0%   163.5n ± 0%       ~ (p=0.283 n=10)
Marshal/Go_struct_many_fields_all_omitempty_all_nonempty_to_CBOR_map   800.4n ± 0%   784.8n ± 0%  -1.94% (p=0.000 n=10)
Marshal/Go_struct_some_fields_all_omitempty_all_nonempty_to_CBOR_map   278.7n ± 0%   275.4n ± 0%  -1.15% (p=0.000 n=10)
Marshal/Go_struct_many_fields_one_omitempty_to_CBOR_map                730.9n ± 0%   727.7n ± 0%  -0.43% (p=0.000 n=10)
Marshal/Go_struct_some_fields_one_omitempty_to_CBOR_map                259.3n ± 0%   257.6n ± 0%  -0.64% (p=0.000 n=10)
Marshal/Go_struct_keyasint_to_CBOR_map                                 1.414µ ± 1%   1.414µ ± 1%       ~ (p=0.445 n=10)
Marshal/Go_struct_toarray_to_CBOR_array                                1.352µ ± 1%   1.338µ ± 1%  -1.07% (p=0.007 n=10)
MarshalCanonical/Go_struct_to_CBOR_map                                 392.5n ± 0%   392.4n ± 0%       ~ (p=0.514 n=10)
MarshalCanonical/Go_struct_to_CBOR_map_canonical                       393.7n ± 0%   384.8n ± 0%  -2.25% (p=0.001 n=10)
geomean                                                                543.3n        540.4n       -0.54%

One benchmark case appeared to regress, but I'm convinced I missed an interfering background process during that run. Re-running that case gave results on par with the previous implementation:

                                                                     │    0.txt    │                1.txt                 │
                                                                     │   sec/op    │   sec/op     vs base                 │
Marshal/Go_struct_many_fields_all_omitempty_all_empty_to_CBOR_map      425.5n ± 0%   425.9n ± 0%  +0.09% (p=0.015 n=10)

[fxamacker]

Thanks @benluddy for updating this PR and sharing benchmarks! 👍 LGTM!