The compilation time is slow

[certik]

I used nlohmann::json in some AST tree to JSON conversion in a compiler, and it was very easy to get started, documentation is good, syntax is simple and it just works. However, I noticed that the compilation time of my code got really slow. So I switched to rapidjson, it took me a lot longer to get started, but when all was done, the compilation of one file got from 4.8s to 1.8s, here is the patch that achieves this speed up:

https://gitlab.com/lfortran/lfortran/commit/904cb6301e86f95e6ad86d571ea74b23cd2b0e70

and another file from 5.7s to 2.8s:

https://gitlab.com/lfortran/lfortran/commit/bfe179ec2ab72855d273db1258c858857ddb9fa2

I am posting it here as a feedback, if there is anything that you can do to speedup the compilation, that would be very helpful.

[volanin]

I just want to +1 this suggestion.

Compilation with nlohmann::json is really, really slow. I use it extensively in my company's project, but since we're in the initial phase of rapid prototyping, the high compilation times are making development grind to a halt.

I found this ticket while searching for a way to improve compilation times with nlohmann::json. Even gcc precompiled headers aren't solving the problem.

[pmbittner]

Something that could help is the forward declaration header. I used it in my project so that I only had to include json in my cpp files.
However, the json_fwd.hpp does not work ad-hoc with the singleinclude json header (you have to remove the default template arguments in json.hpp).

Another suggestion would be to introduce variability, maybe through ifdefs or multiple instead of a single header file. There are lots of cool features in nlohmann::json but as I do not need all of them, turning some things off for compilation would help, too.

[volanin]

I don't think json_fwd.hpp works together with NLOHMANN_JSON_SERIALIZE_ENUM. The enums serialization must be defined in the header file, and it requires json.hpp. Or maybe I'm doing something wrong...

[ClaudiuHKS]

Constexpr slow down compilation time but should help execute code faster.

I am not REALLY sure of what I said above but I hide strings in some of my binaries using Constexpr and I see a huge difference while compiling .. like it takes more time to finish when using Constexpr.

[nlohmann]

@ClaudiuHKS Do you have some benchmarks on the effect of constexpr?

[ClaudiuHKS]

@ClaudiuHKS Do you have some benchmarks on the effect of constexpr?

With Constexpr

#include <stdio.h>
void main (void) { constexpr char Data [8192] = { '-', }; printf ("%s\n", Data); };

Without Constexpr

#include <stdio.h>
void main (void) { char Data [8192] = { '-', }; printf ("%s\n", Data); };

You should really see the difference yourself, while compiling them, like it takes much more time to compile when Constexpr exists, but I think the code will execute faster when it runs, rather than when Constexpr is absent.

NOTE! I have tested it with Visual Studio 2019 on Windows 10, I have no idea if it's the same for other operating systems.

CPPReference states that the constexpr specifier declares that it is possible to evaluate the value of the function or variable at compile time so IMO affecting code at compile time automatically changes code at run time, at least, the way it's being executed.

[nlohmann]

I cannot reproduce this on macOS:

[ClaudiuHKS]

https://pastebin.com/Wbz0rHd6

With Constexpr = Time Elapsed 00:00:02.57
Without Constexpr = Time Elapsed 00:00:00.36

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[fzyzcjy]

Hi, any updates? The slowness of compilation is quite annoying :(

[mcourteaux]

Please reopen. This stale-bot is just closing valid stuff for no reason.

[nlohmann]

It's closed, because nobody was working on this, and this is a hobby project...

[certik]

@nlohmann I see that you gave a thumbs down to my original issue report above. May I ask what I wrote that deserved this? Do you not want me to give feedback anymore? Or was the tone of my report not good (if so, how can I improve it)?