[cfe-dev] zapcc compiler

[James Widman]

On Tue, May 26, 2015 at 1:38 PM, David Blaikie <dblaikie at gmail.com> wrote:
> On Mon, May 25, 2015 at 12:37 PM, Yaron Keren <yaron.keren at gmail.com> wrote:
>>
>> zapcc maintains as much as possible from previous compilations: AST, IR,
>> MC and DebugInfo.  I'm not sure that module support goes that far.
>
>
> ASTs are preserved in modules, that's all they're for (parsing time tends to
> dominate, at least in our world/experiments/data as I understand it, so
> that's the first thing to fix). Duplicate IR/MC/DebugInfo is still present
> though it'd be the next thing to solve - we're talking about deduplicating
> some of the debug info and Adrian Prantl is working on that at the moment -
> putting debug info for types into the module files themselves and
> referencing it directly as a split DWARF file.
>
> Duplicate IR/MC comes from comdat/linkonce_odr functions - and at some point
> it'd be nice to put those in a module too, if there's a clear single
> ownership (oh, you have an inline function in your modular header - OK,
> we'll IRGen it, make an available_externally copy of it in the module to be
> linked into any users of the module, and a standard external definition will
> be codegen'd down to object code and put in the module to be passed to the
> linker). This wouldn't solve the problems with templates that have no 'home'
> to put their definition.

I guess it depends on the build setup:  if you spread the build across
multiple machines then... never mind.

But if the whole build is on one machine and it has enough memory, and
as long as something like zapcc is retaining the whole program's AST
anyway, it could be a win for it to complete that whole-program AST
before any IR is generated.  Presumably, the compiler could then
invent the 'home' and do each instantiation exactly once in the entire
build.

Or... it might still help the multi-machine setup.  In the worst case,
an instantiated function would get instantiated once per machine.

But in that case it might be nice to get a fix-it hint from the linker
to automatically extern-templateize all such instantiations. (:

--James