[webkit-dev] Reducing the use of EncodedJSValue and use JSValue directly instead.

Geoffrey Garen ggaren at apple.com
Tue Jan 3 14:54:33 PST 2017

EncodedJSValue was always just a work-around to convince the compiler to put a JSValue in registers (instead of on the stack, with different compilers disagreeing about where on the stack).

I agree with removing EncodedJSValue if possible.

Did something change to make this possible? For example, are you sure that Windows and 32bit are OK with this change?

I don’t understand why C linkage would affect things: Either the compiler puts structs in registers or it doesn’t.


> On Jan 3, 2017, at 2:44 PM, Filip Pizlo <fpizlo at apple.com> wrote:
> I think that this is great!
> I agree with the policy that we should use JSValue everywhere that it would give us the same codegen/ABI (args in registers, return in registers, etc). 
> -Filip
> On Jan 3, 2017, at 14:33, Mark Lam <mark.lam at apple.com <mailto:mark.lam at apple.com>> wrote:
>> Over the holiday period, I looked into the possibility of giving EncodedJSValue a default constructor (because I didn’t like having to return encodedJSValue() instead of { } in lots of places), and learned why we had EncodedJSValue in the first place (i.e. for C linkage).  This led me to discover (in my reading of the code) that we don’t really need to use EncodedJSValue for most of our host functions (those of type NativeFunction, GetValueFunc, and PutValueFunc).  
>> I propose that we switch to using JSValue directly where we can.  This has the benefit of:
>> 1. better type safety with the use of JSValue (EncodedJSValue is a int64_t typedef).
>> 2. more readable code (less marshaling back and forth with JSValue::encode()/decode()).
>> The patch for this change can be found here:
>> https://bugs.webkit.org/show_bug.cgi?id=166658 <https://bugs.webkit.org/show_bug.cgi?id=166658>
>> Perf is neutral.  Any opinions?
>> Thanks.
>> Mark
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