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Golang port of simdjson: parsing gigabytes of JSON per second
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chart Chart showing performance comparison between simdjson and simdjson-go Feb 4, 2020
examples Fix example import (#2) Feb 11, 2020
testdata Fix serializer & tests (#7) Feb 18, 2020
.gitignore Handy function for Stage 2 debugging Feb 7, 2020
LICENSE Added Apache 2 license Jan 14, 2020 Fix typo (#8) Feb 17, 2020
benchmarks_test.go Fix serializer & tests (#7) Feb 18, 2020
finalize_structurals_amd64.s Make preparations for integrated version to find structural indices (… Oct 1, 2019
find_newline_delimiters_amd64.s AVX512 support for find_newline_delimiters Feb 18, 2020
find_odd_backslash_sequences_amd64.s AVX512 version of find_odd_backslash_sequences Feb 18, 2020
find_quote_mask_and_bits_amd64.s AVX512 version of find_quote_mask_and_bits Feb 20, 2020
find_structural_bits_amd64.s Integrate abs position calculation into stage 1 Jan 23, 2020
find_subroutines_amd64.go AVX512 version of find_quote_mask_and_bits Feb 20, 2020
find_subroutines_amd64_test.go AVX512 version of find_quote_mask_and_bits Feb 20, 2020
find_subroutines_arm64.go Add stub function definitions for arm64 annd ppc64le Feb 16, 2020
find_subroutines_ppc64le.go Add stub function definitions for arm64 annd ppc64le Feb 16, 2020
find_subroutines_s390x.go Stubs for s390x as well Feb 16, 2020
find_whitespace_and_structurals_amd64.s AVX512 version of find_whitespace_and_structurals Feb 19, 2020
flatten_bits_amd64.s Add absolute position calculation to flatten_bits_incremental Jan 23, 2020
go.mod Fix serializer & tests (#7) Feb 18, 2020
go.sum Fix serializer & tests (#7) Feb 18, 2020
ndjson_test.go Fix serializer & tests (#7) Feb 18, 2020
parse_number_amd64.go Add license header Feb 4, 2020
parse_number_amd64.s Framework for parse_number (integer and float) functionality Sep 25, 2019
parse_number_test.go Add missing license headers Feb 5, 2020
parse_string_amd64.go Add license header Feb 4, 2020
parse_string_amd64.s Add maximumStringSize added to parse_string_validate Jan 24, 2020
parse_string_test.go Add jsoniter benchmark (#6) Feb 14, 2020
parsed_array.go Add license header Feb 4, 2020
parsed_json.go Fix serializer & tests (#7) Feb 18, 2020
parsed_json_test.go Fix serializer & tests (#7) Feb 18, 2020
parsed_object.go Simplify and speed up Object FindKey (#9) Feb 18, 2020
parsed_serialize.go Fix serializer & tests (#7) Feb 18, 2020
parsed_serialize_test.go Fix serializer & tests (#7) Feb 18, 2020
simdjson.go Fix racy input buffer (#4) Feb 13, 2020
simdjson_test.go Fix remaining Fail/Pass test cases Feb 7, 2020
stage1_find_marks.go Add jsoniter benchmark (#6) Feb 14, 2020
stage1_find_marks_test.go Fix serializer & tests (#7) Feb 18, 2020
stage2_build_tape.go Add jsoniter benchmark (#6) Feb 14, 2020
stage2_build_tape_test.go Add missing license headers Feb 5, 2020



This is a Golang port of simdjson, a high performance JSON parser developed by Daniel Lemire and Geoff Langdale. It makes extensive use of SIMD instructions to achieve parsing performance of gigabytes of JSON per second.

Performance wise, simdjson-go runs on average at about 40% to 60% of the speed of simdjson. Compared to Golang's standard package encoding/json, simdjson-go is about 10x faster.



simdjson-go is a validating parser, meaning that it amongst others validates and checks numerical values, booleans etc. Therefore these values are available as the appropriate int and float64 representations after parsing.

Additionally simdjson-go has the following features:

  • No 4 GB object limit
  • Support for ndjson (newline delimited json)
  • Proper memory management
  • Pure Go (no need for cgo)

Performance vs simdjson

Based on the same set of JSON test files, the graph below shows a comparison between simdjson and simdjson-go.


These numbers were measured on a MacBook Pro equipped with a 3.1 GHz Intel Core i7. Also, to make it a fair comparison, the constant GOLANG_NUMBER_PARSING was set to false (default is true) in order to use the same number parsing function (which is faster at the expense of some precision; see more below).

Performance vs encoding/json and json-iterator/go

Below is a performance comparison to Golang's standard package encoding/json based on the same set of JSON test files.

$ benchcmp                    encoding_json.txt      simdjson-go.txt
benchmark                     old MB/s               new MB/s         speedup
BenchmarkApache_builds-8      106.77                  948.75           8.89x
BenchmarkCanada-8              54.39                  519.85           9.56x
BenchmarkCitm_catalog-8       100.44                 1565.28          15.58x
BenchmarkGithub_events-8      159.49                  848.88           5.32x
BenchmarkGsoc_2018-8          152.93                 2515.59          16.45x
BenchmarkInstruments-8         82.82                  811.61           9.80x
BenchmarkMarine_ik-8           48.12                  422.43           8.78x
BenchmarkMesh-8                49.38                  371.39           7.52x
BenchmarkMesh_pretty-8         73.10                  784.89          10.74x
BenchmarkNumbers-8            160.69                  434.85           2.71x
BenchmarkRandom-8              66.56                  615.12           9.24x
BenchmarkTwitter-8             79.05                 1193.47          15.10x
BenchmarkTwitterescaped-8      83.96                  536.19           6.39x
BenchmarkUpdate_center-8       73.92                  860.52          11.64x

Also simdjson-go uses less additional memory and allocations.

Here is another benchmark comparison to json-iterator/go:

$ benchcmp                    json-iterator.txt      simdjson-go.txt
benchmark                     old MB/s               new MB/s         speedup
BenchmarkApache_builds-8      154.65                  948.75           6.13x
BenchmarkCanada-8              40.34                  519.85          12.89x
BenchmarkCitm_catalog-8       183.69                 1565.28           8.52x
BenchmarkGithub_events-8      170.77                  848.88           4.97x
BenchmarkGsoc_2018-8          225.13                 2515.59          11.17x
BenchmarkInstruments-8        120.39                  811.61           6.74x
BenchmarkMarine_ik-8           61.71                  422.43           6.85x
BenchmarkMesh-8                50.66                  371.39           7.33x
BenchmarkMesh_pretty-8         90.36                  784.89           8.69x
BenchmarkNumbers-8             52.61                  434.85           8.27x
BenchmarkRandom-8              85.87                  615.12           7.16x
BenchmarkTwitter-8            139.57                 1193.47           8.55x
BenchmarkTwitterescaped-8     102.28                  536.19           5.24x
BenchmarkUpdate_center-8      101.41                  860.52           8.49x


Run the following command in order to install simdjson-go

$ go get

In order to parse a JSON byte stream, you either call simdjson.Parse() or simdjson.ParseND() for newline delimited JSON files. Both of these functions return a ParsedJson struct that can be used to navigate the JSON object by calling Iter().

Using the type Iter you can call Advance() to iterate over the tape, like so:

for {
    typ := iter.Advance()

    switch typ {
    case simdjson.TypeRoot:
        if typ, tmp, err = iter.Root(tmp); err != nil {

        if typ == simdjson.TypeObject {
            if obj, err = tmp.Object(obj); err != nil {

            e := obj.FindKey(key, &elem)
            if e != nil && elem.Type == simdjson.TypeString {
                v, _ := elem.Iter.StringBytes()


More examples can be found in the examples subdirectory and further documentation can be found at godoc.


simdjson-go has the following requirements:

  • A CPU with both AVX2 and CLMUL is required (Haswell from 2013 onwards should do for Intel, for AMD a Ryzen/EPIC CPU (Q1 2017) should be sufficient). This can be checked using the provided SupportedCPU() function.


simdjson-go follows the same two stage design as simdjson. During the first stage the structural elements ({, }, [, ], :, and ,) are detected and forwarded as offsets in the message buffer to the second stage. The second stage builds a tape format of the structure of the JSON document.

Note that in contrast to simdjson, simdjson-go outputs uint32 increments (as opposed to absolute values) to the second stage. This allows arbitrarily large JSON files to be parsed (as long as a single (string) element does not surpass 4 GB...).

Also, for better performance, both stages run concurrently as separate go routines and a go channel is used to communicate between the two stages.

Stage 1

Stage 1 has been converted from the original C code (containing the SIMD intrinsics) to Golang assembly using c2goasm. It essentially consists of five separate steps, being:

  • find_odd_backslash_sequences: detect backslash characters used to escape quotes
  • find_quote_mask_and_bits: generate a mask with bits turned on for characters between quotes
  • find_whitespace_and_structurals: generate a mask for whitespace plus a mask for the structural characters
  • finalize_structurals: combine the masks computed above into a final mask where each active bit represents the position of a structural character in the input message.
  • flatten_bits_incremental: output the active bits in the final mask as incremental offsets.

For more details you can take a look at the various test cases in find_subroutines_amd64_test.go to see how the individual routines can be invoked (typically with a 64 byte input buffer that generates one or more 64-bit masks).

There is one final routine, find_structural_bits_in_slice, that ties it all together and is invoked with a slice of the message buffer in order to find the incremental offsets.

Stage 2

During Stage 2 the tape structure is constructed. It is essentially a single function that jumps around as it finds the various structural characters and builds the hierarchy of the JSON document that it processes. The values of the JSON elements such as strings, integers, booleans etc. are parsed and written to the tape.

Any errors (such as an array not being closed or a missing closing brace) are detected and reported back as errors to the client.

Tape format

Similarly to simdjson, simdjson-go parses the structure onto a 'tape' format. With this format it is possible to skip over arrays and (sub)objects as the sizes are recorded in the tape.

simdjson-go format is exactly the same as the simdjson tape format with the following 2 exceptions:

In order to support ndjson, it is possible to have many root elements on the tape. Also, to allow for fast navigation over root elements, a root points to the next root element (and as such the last root element points 1 index past the length of the tape).

Strings are handled differently, unlike simdjson the string size is not prepended in the String buffer but is added as an additional element to the tape itself (much like integers and floats). Only strings that contain special characters are copied to the String buffer in which case the payload from the tape is the offset into the String buffer. For string values without special characters the tape's payload points directly into the message buffer.

For more information, see TestStage2BuildTape in stage2_build_tape_test.go.

Minor number inprecisions

The number parser has minor inprecisions compared to Golang's standard number parsing. There is constant GOLANG_NUMBER_PARSING (on by default) that uses Go's parsing functionality at the expense of giving up some performance. Note that the performance metrics mentioned above have been measured by setting the GOLANG_NUMBER_PARSING to false.

Fuzz Tests

simdjson-go has been extensively fuzz tested to ensure that input cannot generate crashes and that output matches the standard library.

The fuzzers and corpus are contained in a separate repository at

The repo contains information on how to run them.


simdjson-go is released under the Apache License v2.0. You can find the complete text in the file LICENSE.


Contributions are welcome, please send PRs for any enhancements.

If your PR include parsing changes please run fuzz testers for a couple of hours.

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