1422040 - Add the ability to use ipc::ByteBuf for blob image transfer

Status: RESOLVED WONTFIX

(Core :: Graphics: WebRender, enhancement, P2)

Description

[Jeff Muizelaar [:jrmuizel]]

This will let us avoid bug 1405824 and might be fast enough that we don't need to worry about not using shared memory for now.

Comment 1

[Jeff Muizelaar [:jrmuizel]]

In this profile https://perfht.ml/2ApmBjH shared memory allocation is taking up at least 10% of webrender display list creation time

Comment 2

[Nicolas Silva [:nical]]

I might have let out that this was straight forward during yesterday's meeting but in fact IPDL still can't handle move-only types unless they are passed directly as a parameter of the message (so no way to pass it in an ipdl union or struct, which we can't do without in any reasonable way here).

So I am this close to go face the undefined behavior dragons by having ByteBuffer fake a copy constructor that const_casts the source and nulls out the buffer (making it equivalent to a move).

Comment 3

[Nicolas Silva [:nical]]

Hm, nevermind, this code in the ipdl compiler:

> def _cxxTypeNeedsMove(ipdltype):
>    return ipdltype.isIPDL() and (ipdltype.isArray() or
>                                  ipdltype.isShmem() or
>                                  ipdltype.isByteBuf() or
>                                  ipdltype.isEndpoint())

Refers to ByteBuf which is not ByteBuffer.

Comment 4

[Nicolas Silva [:nical]]

After a bit more than a day of refactoring and going through compile errors, the current status is:

- IPDL doesn't support move-only types in structures and enum (only directly as message arguments)
- IPDL doesn't support serializing nsTArrays of move-only types either due to the array being passed as a const reference while serializing (see generated PFoo::Write(const nsTarray<ByteBuf>&, Message*)) which requires the ParamTrait::Write to be const (which we explicitly don't want here).

Comment 5

[Nicolas Silva [:nical]]

This got replaced by bug 1405824 (for now) due to IPDL's allergy to non-copyable types.

Comment 6

[Nicolas Silva [:nical]]

I don't plan to work on this any time soon and bug 1405824 was fixed some other way so closing.