[cfe-dev] Clang 3.5 Release Pre-Pre-Pre-Announcement

[Reid Kleckner]

On Tue, May 20, 2014 at 3:54 PM, Alp Toker <alp at nuanti.com> wrote:

>
> On 19/05/2014 04:41, Rafael Espíndola wrote:
>
>> TLDR; We _really_ want to calculate these dependencies at build time with
>>> LLVMBuild instead of annotating the source.
>>>
>> Is there a convenient way to feed this to link.exe?
>>
>
> We can pass in exports through a /def file, similar to exports with other
> linkers.
>
> To calculate the exports, someone will have to write a script/tool that
> collects undefined symbols, then goes through visible symbols in other
> modules to derive the export lists.
>
> That'll generate minimal export lists, tuned for each platform and
> configuration, permitting optimal dead stripping and optimisation. And
> it'll save us from the burden of annotating loads and loads of definitions
> in the source that only work for a few setups.


It's an interesting idea, but I'm pretty opposed to this approach.

First, this approach doesn't work for exported data.  You need annotations
to import data, and I can say from experience that we do this in LLVM.

Second, I tried to rig up something that exports all symbols with dumpbin
once and it was ridiculously slow.  I think anything that tried to scrape
imports of one set of object files from another would also be really slow.
 Even if we used llvm-nm and made it fast, it'd be part of our build
system.  Ew.

Third, suppose we want to build LLVMSupport.dll.  Now we need to build
LLVMSupport.lib, then everything that uses LLVMSupport.dll, and then scrape
imports from all those static archives or object files.  This is a *huge*
web of dependencies.  Now building llvm-as.exe which dynamically links
LLVMSupport.dll depends on building clangSema.lib.  That's a non-starter.

If we want to support DSOs exporting a C++ interface, IMO we should define
the DSO boundary up front and use annotations.  I think the logical DSO
boundaries would be LLVMSupport, LLVM, and Clang.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20140520/849045c1/attachment.html>