[llvm-dev] RFC: LTO should use -disable-llvm-verifier

Peter Collingbourne via llvm-dev
Fri, 04 Sep 2015 22:53:08 -0000

On Fri, Sep 04, 2015 at 03:11:39PM -0700, Mehdi Amini
wrote:
>
> > On Sep 4, 2015, at 11:38 AM, Peter Collingbourne <peter at
pcc.me.uk> wrote:
> >
> > On Fri, Sep 04, 2015 at 11:13:43AM -0700, Mehdi Amini wrote:
> >>
> >>> On Sep 4, 2015, at 11:03 AM, Eric Christopher <echristo at
gmail.com> wrote:
> >>>
> >>>
> >>>
> >>> On Fri, Sep 4, 2015 at 12:48 AM Mehdi Amini <mehdi.amini at
apple.com <mailto:mehdi.amini at apple.com>> wrote:
> >>>> On Sep 4, 2015, at 12:22 AM, Eric Christopher <echristo
at gmail.com <mailto:echristo at gmail.com>> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On Thu, Sep 3, 2015 at 11:45 PM Mehdi Amini
<mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
> >>>> Hi,
> >>>>
> >>>>> On Sep 2, 2015, at 7:31 PM, Peter Collingbourne via
llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>> wrote:
> >>>>>
> >>>>> On Thu, Sep 03, 2015 at 01:10:42AM +0000, Eric
Christopher wrote:
> >>>>>> On Tue, Sep 1, 2015 at 10:43 AM Duncan P. N. Exon
Smith <
> >>>>>> dexonsmith at apple.com <mailto:dexonsmith at
apple.com>> wrote:
> >>>>>>
> >>>>>>>
> >>>>>>>> On 2015-Aug-31, at 18:09, Eric Christopher
<echristo at gmail.com <mailto:echristo at gmail.com>> wrote:
> >>>>>>>>
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> On Mon, Aug 31, 2015 at 5:50 PM Duncan P.
N. Exon Smith <
> >>>>>>> dexonsmith at apple.com <mailto:dexonsmith
at apple.com>> wrote:
> >>>>>>>>
> >>>>>>>>> On 2015-Aug-31, at 12:21, Eric
Christopher <echristo at gmail.com <mailto:echristo at gmail.com>>
wrote:
> >>>>>>>>> Yep. This is where I was going :)
> >>>>>>>>
> >>>>>>>> Glad I found consensus, but I want to
double-check that this makes
> >>>>>>>> sense to add to the driver.  I didn't
quite think through the
> >>>>>>>> implications myself.
> >>>>>>>>
> >>>>>>>> Since the driver doesn't know if
there's any bitcode, or if LTO is
> >>>>>>>> going to be invoked, it seems like
I'll have to change the noasserts
> >>>>>>>> driver to *always* pass the option to the
linker just in case we are
> >>>>>>>> doing LTO.  Is this reasonable?
> >>>>>>>>
> >>>>>>>> Also, I realized that passing `-mllvm
-disable-llvm-verifier` to ld64
> >>>>>>>> is redundant... so I'm thinking
`-mllvm -disable-verify`.  Make
> >>>>>>>> sense?
> >>>>>>>>
> >>>>>>>> *sigh* Reasons to hate the driver
interface again...
> >>>>>>>>
> >>>>>>>> I guess this is ok. Could possibly add it
to the existing terrible
> >>>>>>> "enable this pass" interface on
liblto as well.
> >>>>>>>
> >>>>>>> The linker doesn't know whether clang was
built with asserts, though.
> >>>>>>>
> >>>>>>> We could just make it implicit: move the
decision to libLTO itself.  Given
> >>>>>>> that clang and libLTO.dylib are different
executables anyway -- and you
> >>>>>>> might be interposing an asserts libLTO.dylib
to use with an installed clang
> >>>>>>> -- maybe it's even better?
> >>>>>>>
> >>>>>>>
> >>>>>> *nod* We could do that. Seems better than the
alternative.
> >>>>>
> >>>>> +1
> >>>>>
> >>>>>>>> I don't suppose ld64 could move to a
model like we're talking about with
> >>>>>>> lld that pcc is working on?
> >>>>>>>
> >>>>>>> What specifically?
> >>>>>>
> >>>>>>
> >>>>>> Ah, using the C++ interface to handle everything
and not using libLTO at
> >>>>>> all.
> >>>>>>
> >>>>>> He can speak more to this though.
> >>>>>
> >>>>> The C++ interface is much more convenient for a C++
program to use, but
> >>>>> clients need to revlock themselves to LLVM in order to
use it.
> >>>>>
> >>>>> In theory the same does not apply to libLTO, but we do
end up changing it
> >>>>> occasionally to add new APIs which ld64 promptly
starts using, so it isn't
> >>>>> clear how much ld64 gains by relying on libLTO,
> >>>>
> >>>> Backward/Forward compatibility?
> >>>> Drop any version of clang/libLTO and still be able to use
the system provided linker on any version of OS X?
> >>>> Sounds like a valuable feature to me, which is what I
believe was (is?) sought by the C API in general (but that’s another story).
> >>>>
> >>>>
> >>>> Static linking against the parts of llvm you care about?
There's nothing in the ld64 build system that means it can't do this :)
> >>>
> >>> I’m not sure how is that supposed to work?
> >>> I drop a new clang/libLTO on the system, clang generates
bitcode, and libLTO handles it.
> >>>
> >>> If ld64 was statically linked against LLVM, it couldn’t read
the new bitcode right?
> >>>
> >>>
> >>> Well sure, that's what Peter meant when he said revlocked.
:)
> >>
> >> I’m not sure we’re talking about the same thing, I may have
misunderstood his point (“In theory the same does not apply to libLTO, but we do
end up changing it
> >> occasionally to add new APIs which ld64 promptly starts using, so
it isn’t clear how much ld64 gains by relying on libLTO”) ; my take from that
was that since the new ld64 starts using the new libLTO interface, there is a
revlock anyway and then there is no advantage in keeping the old libLTO entry
points.
> >> I just described a use case that shows how useful it can be.
> >>
> >> Apologize if I misunderstood the point.
> >
> > That seems like a potentially valid use case, but if it doesn't
actually
> > work right now it doesn't really buy you much.
>
> It is working right now AFAIK. At least my understand is that the current
situation is that you can drop a clang/libLTO built today on a machine that has
ld64 from a few years ago and it will work.

Is there anyone who actually needs to do this? Why can't they upgrade
their linker?

> > If this is indeed an abolute requirement, there are better ways of
achieving
> > this kind of thing without creating an API stability burden in LLVM.
For
> > example you could make libLTO a build product of ld64 rather than of
LLVM by
> > copying the tools/lto code into ld64 and using the preprocessor to
make the
> > code LLVM-version-dependent. This seems like it could be relatively
easily
> > achievable right now; all you'd need to do is switch the Xcode
toolchain
> > over to shipping the libLTO build product from ld64 instead of from
LLVM
> > (or even statically linking as Eric suggested). Any other existing
clients
> > of libLTO who care about this can do the same thing.
>
> It seems I’m missing something: the system ld64 linker will be shipped with
a self-contained libLTO, but the new compiler I’ll drop-in will generate
“new" bitcode that will be fed to this “old” libLTO, and that’s not gonna
play well.
> Unless you are only including the C API wrapper in ld64, but in this case
I’m not sure I understand "using the preprocessor to make the code
LLVM-version-dependent”.

Yes, I mean only the C API wrapper, but the code would look like this:

void lto_do_something() {
#if llvm_version > 123
  use_new_api();
#else
  use_old_api();
#endif
}

for however many versions of LLVM you need to care about.

> You will be able to build ld64 independently of LLVM, but how about at
runtime? The compiler needs to ship a dynamic library that will be loaded by the
linker, and there is still a need for a stable API there.

There's no reason why the workflow needs to be "drop in some libLTO
component
built by LLVM on its own". If I'm doing something as unusual as
building
my own LLVM which I want to use with LTO it isn't so much of an additional
burden to build some other component to make LTO work with my linker. That
other component would live in the ld64 source tree and would either be ld64
itself or what is currently the libLTO DSO.

Thanks,
--
Peter