Hello WebKittens,

I recently striped 830KB binary size in WebKit just by using a work-around.
This email describes what happened so far, to prevent from happening again.

## Problem

When C/C++ struct/class is included in field types and method types in 
Objective-C, Objective-C compiler puts type-enconding-string which gathers type 
information one-leve deep for C/C++ struct/class if

1. The type is a pointer to C/C++ struct/class
2. The type is a value of C/C++ struct/class
3. The type is a reference to C/C++ struct/class

However, our WebKit C/C++ struct/class is typically very complex type using a 
lot of templates. Unfortunately, Objective-C compiler includes expanded 
template definition as a string and adds it as a type-enconding-string into the 
release binary!

For example, https://trac.webkit.org/changeset/254152/webkit 
<https://trac.webkit.org/changeset/254152/webkit> is removing JSC::VM& from 
Objective-C signature, and it reduces 200KB binary size!
Another example is https://trac.webkit.org/changeset/254241/webkit 
<https://trac.webkit.org/changeset/254241/webkit>, which removes a lot of 
WebCore::WebView* etc. from Objective-C method signature, and reduces 630KB 
binary.

## Solution for now

We can purge type-encoding-string if we use Objective-C NS_DIRECT feature 
(which makes Objective-C function as C function calling convention, removing 
metadata).
However, this does not work universally: with NS_DIRECT, Objective-C override 
does not work. This means we need to be extra-careful when using it.

So, as a simple, but effective work-around, in the above patch, we introduced 
NakedRef<T> / NakedPtr<T>. This is basically raw pointer / raw reference to T, 
with a wrapper class.
This leverages the behavior of Objective-C compiler’s mechanism “one-level deep 
type information collection”. Since NakedRef<T> / NakedPtr<T> introduces 
one-level deep field,
Objective-C compiler does not collect the type information of T if NakedPtr<T> 
is included in the fields / signatures, while the compiler collects information 
when T* is used.

So, if you are using T& / T* C/C++ struct/class in Objective-C, let’s convert 
it to NakedRef<T> / NakedPtr<T>. Then you could save much binary size 
immediately without causing any performance problem.

## Future work

We would like to avoid including such types accidentally in Objective-C. We 
should introduce build-time hook script which detects such a thing.
I uploaded the PoC script in https://bugs.webkit.org/show_bug.cgi?id=205968 
<https://bugs.webkit.org/show_bug.cgi?id=205968>, and I’m personally planning 
to introduce such a hook into a part of build process.


-Yusuke
_______________________________________________
webkit-dev mailing list
webkit-dev@lists.webkit.org
https://lists.webkit.org/mailman/listinfo/webkit-dev

Reply via email to