[webkit-dev] JavaScriptCore performance is very slow on Windows

Eric Wing ewmailing at gmail.com
Mon Jun 29 09:51:54 PDT 2015

I’ve been trying to use JavaScriptCore as an embedded scripting
language for a video game engine. (Note: I am just using JSCore, not
WebKit.) I’ve been using it on multiple platforms. During my testing,
I’ve discovered that the JavaScriptCore Windows performance is very
slow. I wanted to alert you in hopes there is something that can be
done to fix it.

I’ve been getting help on building JavaScriptCore for Windows from a
few people in the community. My current build is as of a couple of
weeks ago. However, I’ve been seeing the same performance
characteristics from a build from a little over a year ago.

To prove to you that this is a problem to just JavaScriptCore on
Windows, and not a general Windows problem, nor a problem with my
code, I have done the following:

- I ran the program on Windows, OS X, Linux, iOS, Android, and
Raspberry Pi (Linux).
- I wrote comparable programs in C and Lua and also ran these on all platforms.
- Additionally, I was able to run on the same hardware (dual boot) for

So here is the quick summary:

                   Ubuntu 12.04 LTS (64-bit)
Windows 8.1 (64-bit)
C                     5600 sprites
         27000 sprites
Lua                  3100 sprites
       12000 sprites
JSCore            2400 sprites
     120 sprites

This was on an Intel Core i5-3570K @ 3.40 GHz with Intel 4000 HD
graphics. The reason for the big jump in Windows performance is mostly
due to the Intel graphics drivers being poor on Linux. However, that
120 sprites is not a typo. Whereas everything else was faster on
Windows, JSCore is magnitudes slower.

My numbers for the other operating systems tend to follow sensible
trends. Windows/JSCore is an outlier. And other programs I’ve run also
show JavaScriptCore on Windows has performance problems.

You can see this summary with the programs running in a short video I
put together and uploaded to YouTube:

I have uploaded pre-built binaries of my test program. JavaScriptCore
is dynamically linked in all cases, so it is easy to swap in another
build of JSCore if you want to try things. There is a file called
config.txt in each bundle that lets you change the number of sprites.
(I did not sign the OS X binaries with my Developer ID because I
figured you might want to change the config.txt.) Also, the Lua and
JavaScript scripts are in plain text and can be easily modified.

And for clarity: All my test programs here are 64-bit versions. I have
not actually benchmarked 32-bit except on ARM devices.

Windows 7 and up (x64)

OS X 10.9 and up

Linux (x64 Ubuntu 12.04 LTS a.k.a Steam Runtime)

I also made the repos to my programs available:


However, it probably won’t be of much use without my SDK because
without it, you’ll need to compile all the dependencies yourself and
figure out the build system. If you need a copy of my SDK, let me


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