[llvm-dev] ThinLTO and the C API

[Peter Collingbourne via llvm-dev]

That's correct. With the old-format bitcode a couple of features won't work
with ThinLTO (i.e. control flow integrity and whole-program
devirtualization), but presumably you aren't already using them.

Peter

On Thu, Jul 13, 2017 at 11:27 AM, Robinson, Paul <paul.robinson at sony.com>
wrote:

> Thanks Peter!  I think PS4 will have to follow what Darwin does here, at
> least for now.
>
> I assume the new API would consume old-format bitcode with no problem, if
> we decide to make that conversion and still want to read old-style bitcode
> files.
>
> --paulr
>
>
>
> *From:* Peter Collingbourne [mailto:peter at pcc.me.uk]
> *Sent:* Wednesday, July 12, 2017 2:55 PM
> *To:* Robinson, Paul; Mehdi AMINI
> *Cc:* llvm-dev at lists.llvm.org
> *Subject:* Re: [llvm-dev] ThinLTO and the C API
>
>
>
> Hi Paul,
>
>
>
> The current C LTO API wraps a legacy C++ implementation of the LTO API,
> which does not support multi-module bitcode files, and we do not intend to
> teach it to do so. We planned to replace the old API with a new C API that
> wraps the new resolution-based C++ API, but that hasn't happened yet.
>
>
>
> Mehdi was working on the new C API here: https://reviews.llvm.org/D31898
>
> I'm not sure if he will have time to finish it, though.
>
>
>
> To deal with Apple's linker the clang driver passes a flag to the frontend
> on Darwin which prevents it from creating multi-module bitcode files.
>
> http://llvm-cs.pcc.me.uk/tools/clang/lib/Driver/ToolChains/Clang.cpp#2073
>
>
>
> Peter
>
>
>
> On Wed, Jul 12, 2017 at 2:29 PM, Robinson, Paul via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Our ThinLTO testing was inoperative for a while, and now that it's running
> again, we're seeing the linker step reporting "Expected a single module."
> Note that our linker is using the C API to read bitcode.
>
> It looks like the ThinLTO writer now emits two modules, but AFAICT most of
> the APIs in BitcodeReader.cpp expect to see only one.  Is this just an
> oversight in supporting ThinLTO from the C API?
>
> I am a little bemused because I had thought Apple's "ld64" also used the
> C API and supported ThinLTO.
>
> Any help/guidance/tips welcome (other than "switch to the C++ API").
> Thanks,
> --paulr
>
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>
>
>
>
> --
>
> --
>
> Peter
>



-- 
-- 
Peter
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