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frankw d175d963fe Simplify code and pointer arithmetic 1 month ago
chart Chart showing performance comparison between simdjson and simdjson-go 3 months ago
examples Separate platform specific code (#11) 3 months ago
testdata Fix serializer & tests (#7) 3 months ago
.gitignore Handy function for Stage 2 debugging 3 months ago
LICENSE Added Apache 2 license 4 months ago Server info for AVX512 performance measurements 3 months ago
benchmarks_test.go Separate platform specific code (#11) 3 months ago
common.h Move error_mask to K register 2 months ago
finalize_structurals_amd64.s Move structurals to separate K register 3 months ago
find_newline_delimiters_amd64.s Separate platform specific code (#11) 3 months ago
find_odd_backslash_sequences_amd64.s Separate platform specific code (#11) 3 months ago
find_quote_mask_and_bits_amd64.s Simplify loading/storing K registers 2 months ago
find_structural_bits_amd64.s Separate platform specific code (#11) 3 months ago
find_structural_bits_avx512_amd64.s Simplify loading/storing K registers 2 months ago
find_subroutines_amd64.go Move error_mask to K register 2 months ago
find_subroutines_amd64_test.go Add test cases for correct modification of prev_iter_inside_quote state across masks 2 months ago
find_whitespace_and_structurals_amd64.s Simplify loading/storing K registers 2 months ago
flatten_bits_amd64.s Correct flatten_bits to write DWORD 2 months ago
go.mod Fix serializer & tests (#7) 3 months ago
go.sum Fix serializer & tests (#7) 3 months ago
ndjson_test.go Separate platform specific code (#11) 3 months ago
parse_json_amd64.go Separate platform specific code (#11) 3 months ago
parse_json_amd64_test.go Separate platform specific code (#11) 3 months ago
parse_number_amd64.go Separate platform specific code (#11) 3 months ago
parse_number_amd64.s Separate platform specific code (#11) 3 months ago
parse_number_test.go Separate platform specific code (#11) 3 months ago
parse_string_amd64.go Simplify code and pointer arithmetic 1 month ago
parse_string_amd64.s Separate platform specific code (#11) 3 months ago
parse_string_test.go Separate platform specific code (#11) 3 months ago
parsed_array.go Add license header 3 months ago
parsed_json.go Separate platform specific code (#11) 3 months ago
parsed_json_test.go Separate platform specific code (#11) 3 months ago
parsed_object.go Create destination if nil (#12) 2 months ago
parsed_serialize.go Fix serializer & tests (#7) 3 months ago
parsed_serialize_test.go Separate platform specific code (#11) 3 months ago
simdjson_amd64.go Separate platform specific code (#11) 3 months ago
simdjson_amd64_test.go Separate platform specific code (#11) 3 months ago
simdjson_other.go Separate platform specific code (#11) 3 months ago
stage1_find_marks_amd64.go Eliminate unused parameters for "slice" versions of find_structural_bits 2 months ago
stage1_find_marks_amd64_test.go Use bits.Reverse in lieu of reverseBinary 2 months ago
stage2_build_tape_amd64.go Separate platform specific code (#11) 3 months ago
stage2_build_tape_amd64_test.go Separate platform specific code (#11) 3 months ago



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

AVX512 Acceleration

Stage 1 has been optimized using AVX512 instructions. Under full CPU load (8 threads) the AVX512 code is about 1 GB/sec (15%) faster as compared to the AVX2 code.

benchmark                                   AVX2 MB/s    AVX512 MB/s     speedup
BenchmarkFindStructuralBitsParallelLoop      7225.24      8302.96         1.15x

These benchmarks were generated on a c5.2xlarge EC2 instance with a Xeon Platinum 8124M CPU at 3.0 GHz.


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.