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

Fri Sep 4 16:04:28 PDT 2015

> On Sep 4, 2015, at 3:53 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
> 
> 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.

Oh I think I got it, basically you ask about extracting the libLTO C wrapper outside of LLVM as a separate project, and up to us (or anyone who cares about C API stability) to maintain it up to date with respect to the current version of LLVM. And anyone that want to target LTO on an old system would have to checkout the version of this wrapper that matches the LLVM they are targeting, and then build and ship this wrapper alongside with LLVM.
Did I get it right?

Thanks,

— 
Mehdi