[llvm-dev] [lldb-dev] Trying out lld to link windows binaries (using msvc as a compiler)

Leonardo Santagada via llvm-dev llvm-dev at lists.llvm.org
Mon Jan 29 09:24:41 PST 2018

I cleaned up my tests and figured that the obj file generated with problems
was only with msvc 2015, so trying again with msvc 2017 I get:

lld-link: 4s
lld-link /debug: 1m30s and ~20gb of ram
lld-link /debug:ghash: 59s and ~20gb of ram
link: 13s
link /debug:fastlink: 43s and 1gb of ram
link specialpdb: 1m10s and 4gb of ram
link /debug: 9m16s min and >14gb of ram

link incremental: 8s when it works.

*specialpdb is created with passing to a set of compilation units (eg a
folder) the same pdb to be written to, so it dedups the symbols before the
final linking, but that does decrease the concurrency as this step can't be
done after linking.

My question is, in the set of patches you guys haven't upstreamed is there
anything that makes compilation uses less memory? Or just asking more
directly, when will those patches make to upstream, or can I try them? The
memory usage of lld-link is a little worrying as we have around 6-8
binaries that we link for windows and they mostly use the same libraries so
20gb of ram each means we probably can't link them all together anymore.

Tomorrow I will send my tool and changes to lld so more people can try this
out and tell if it helps with their msvc only code.

On Sun, Jan 28, 2018 at 11:22 PM, Zachary Turner <zturner at google.com> wrote:

> I don’t have pgo numbers. When I build using -flto=thin the link time is
> significantly faster than msvc /ltcg and runtime is slightly faster, but I
> haven’t tested on a large variety of different workloads, so YMMV. Link
> time will definitely be faster though
> On Sun, Jan 28, 2018 at 2:20 PM Leonardo Santagada <santagada at gmail.com>
> wrote:
>> This part is only for objects with /Z7 debug information in them right? I
>> think most of the third parties are either: .lib/obj without debug
>> information, the same with information on pdb files. Rewriting all
>> .lib/.obj with /Z7 information seems doable with a small python script, the
>> pdb one is going to be more work, but I always wanted to know how a pdb
>> file is structured so "fun" times ahead. But yeah printing it out, and
>> timing it might be very useful indeed.
>> Did anyone tried to compile/link lld-link.exe with LTO+PGO to see how
>> much faster can it get? I might try that as well, as 10% speed improvement
>> might be handy.
>> On Sun, Jan 28, 2018 at 11:14 PM, Zachary Turner <zturner at google.com>
>> wrote:
>>> Look for this code in lld/coff/pdb.cpp
>>> if (Config->DebugGHashes) {
>>> ArrayRef<GloballyHashedType> Hashes;
>>> std::vector<GloballyHashedType> OwnedHashes;
>>> if (Optional<ArrayRef<uint8_t>> DebugH = getDebugH(File))
>>> Hashes = getHashesFromDebugH(*DebugH);
>>> else {
>>> OwnedHashes = GloballyHashedType::hashTypes(Types);
>>> Hashes = OwnedHashes;
>>> }
>>> In the else block there, add a log message that says “synthesizing
>>> .debug$h section for “ + Obj->Name
>>> See how many of these you get. When I build chrome + all third party
>>> libraries this way i get about 100, which is small enough to still see
>>> large performance gains.
>>> If you have many 3rd party libraries, it may be necessary to rewrite the
>>> .lib files too, not just the .obj files. Eventually I’ll get around to
>>> implementing all of this as well, as well as better heuristics in lld-link
>>> to disable ghash if it’s going to be slow
>>> On Sun, Jan 28, 2018 at 1:51 PM Leonardo Santagada <santagada at gmail.com>
>>> wrote:
>>>> Ok I went for kind of middle ground solution, I patch in the obj files,
>>>> but as adding a new section didn't seem to work, I add a "shadow" section,
>>>> by editing the pointer to line number and the virtual size on the .debug$T
>>>> section. Although technically broken, both link.exe and lld-link.exe don't
>>>> seem to mind the alterations and as the shadow .debug$H is not really a
>>>> section anymore (its just some bytes at the end of the file) it doesn't
>>>> change anything else that does matter. With that I could do my first test
>>>> with a subset of our code base, and the results are not good. I found one
>>>> of our sources that break the ghash computation, I will get more info on
>>>> this and post a proper bug report, but I guess its type information that is
>>>> generated only by msvc. The other more alarming problem is that linking is
>>>> way slower with the ghahes... my guess is that we have a bunch of pdb files
>>>> for some third party libraries and calculating those ghashes takes more
>>>> time than actual linking of this small part of the source (it links in 4s
>>>> in both link.exe and lld-link.exe without ghashes).
>>>> On Fri, Jan 26, 2018 at 8:52 PM, Leonardo Santagada <
>>>> santagada at gmail.com> wrote:
>>>>> We don't generate any .lib as those don't work well with incremental
>>>>> linking (and give zero advantages when linking AFAIK), and it would be
>>>>> pretty easy to have a modern format for having a .ghash for multiple files,
>>>>> something simple like size prefixed name and then size prefixed ghash blobs.
>>>>> On Fri, Jan 26, 2018 at 8:44 PM, Zachary Turner <zturner at google.com>
>>>>> wrote:
>>>>>> We considered that early on, but most object files actually end up in
>>>>>> .lib files so unless there were a way to connect the objects in the .lib to
>>>>>> the corresponding .ghash files, this would disable ghash usage for a large
>>>>>> amount of inputs. Supporting both is an option, but it adds a bit of
>>>>>> complexity and I’m not totally convinced it’s worth it
>>>>>> On Fri, Jan 26, 2018 at 11:38 AM Leonardo Santagada <
>>>>>> santagada at gmail.com> wrote:
>>>>>>> it does.
>>>>>>> I just had an epiphany: why not just write a .ghash file and have
>>>>>>> lld read those if they exist for an .obj file?
>>>>>>> Seem much simpler than trying to wire up a 20 year old file format.
>>>>>>> I will try to do this, is something like this acceptable for LLD? The cool
>>>>>>> thing is that I can generate .ghash for .lib or any obj lying around (maybe
>>>>>>> even for pdb in the future).
>>>>>>> On Fri, Jan 26, 2018 at 8:32 PM, Zachary Turner <zturner at google.com>
>>>>>>> wrote:
>>>>>>>> In general, we should be able to accept any MSVC .obj file to LLD.
>>>>>>>> At the very least, we're not aware of any cases that don't work.
>>>>>>>> Does your MSVC .obj file link fine before you add the .debug$H?
>>>>>>>> On Fri, Jan 26, 2018 at 11:23 AM Leonardo Santagada <
>>>>>>>> santagada at gmail.com> wrote:
>>>>>>>>> Okay, apparently coff2yaml and yaml2coff are not in a great place
>>>>>>>>> as they both don't deal well with the fact that you can have overlapping
>>>>>>>>> sections, which seems to be what clang-cl produces (the .data section
>>>>>>>>> points to the same place as a later section). Which is not a big big
>>>>>>>>> problem for me particularly because msvc doesn't even generate .data
>>>>>>>>> sections in .obj.
>>>>>>>>> I'm trying to put support for .bss sections in both coff2yaml and
>>>>>>>>> yaml2coff... but I still can link just fine with my transformations
>>>>>>>>> clang-cl generated files... what does give me problems is msvc .obj files.
>>>>>>>>> Have you tried to link one of these?
>>>>>>>>> On Fri, Jan 26, 2018 at 8:05 PM, Leonardo Santagada <
>>>>>>>>> santagada at gmail.com> wrote:
>>>>>>>>>> yeah, apparently .bss has a flag of unitialized data that is not
>>>>>>>>>> being respected on the layout of the coff files (it should skip those
>>>>>>>>>> sections) but I dunno what to do with .data as it doesn't have a size.
>>>>>>>>>> (resending as apparently my pastes generated a ton of hidden html
>>>>>>>>>> data and this message hit the mailinglist limit of 100k)
>>>>>>>>>> --
>>>>>>>>>> Leonardo Santagada
>>>>>>>>> --
>>>>>>>>> Leonardo Santagada
>>>>>>> --
>>>>>>> Leonardo Santagada
>>>>> --
>>>>> Leonardo Santagada
>>>> --
>>>> Leonardo Santagada
>> --
>> Leonardo Santagada


Leonardo Santagada
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