[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
Tue Jan 30 09:21:48 PST 2018

Today I played around replacing the sha1 with xxHash64 and the results so
far are bad. Linking times almost doubled and I can't really explain why,
the only thing that comes to mind is hash collisions but on type names they
should be very few in 64bit hashes.

Any reason why you are trying blake2 and not murmurhash3 or xxHash64?

About creating a pdb per lib, you can say to msvc to put the pdb of every
.obj compilation to the same file, but you can't after 20 files compiled to
.obj (with /Z7 or /Zi) to them merge all the debug information in one .pdb
file AFAIK. That would make our links much faster I think as people either
are changing headers (and then they know they have to wait) or changing a
single/few .cpp files. It would be great to group our 3k obj debug
information in groups so that this linking steps can be paralelizable. Is
there any support maybe for merging pdb with pdb util and then feeding that
to lld-link instead of .obj debug info?

I also re-read the post about ghash and it says blink links in 88s, the 28s
you talk about is with unrelased optimizations only?

On Tue, Jan 30, 2018 at 5:54 AM, Zachary Turner <zturner at google.com> wrote:

> You can make a PDB per lib (consider msvcrtd.pdb which ships with MSVC),
> but all these per-lib PDBs would have to be merged into a single master PDB
> at the end, so you still can't avoid that final .  In a way, that's similar
> to the idea behind /DEBUG:FASTLINK (keep the debug info in object files to
> eliminate the cost of merging types and symbol records) and we know what
> the problems with /DEBUG:FASTLINK are.
> The PDB generation code in LLD is still completely single threaded, so
> that's one area for huge potential gains, but only some parts of the
> algorithm are parallelizable.  We're trying to squeeze every last bit of
> performance out of the single-threaded case first before we parallelize,
> but that option is definitely still there for us.
> On Mon, Jan 29, 2018 at 4:35 PM Leonardo Santagada <santagada at gmail.com>
> wrote:
>> Does packing obj files in .lib helps linking in any way? My understanding
>> is that there would be no difference. It could help if I could make a pdb
>> per lib, but there is no way to do so... Maybe we could implement this on
>> lld?
>> On 29 Jan 2018 22:14, "Zachary Turner" <zturner at google.com> wrote:
>>> Yes we've discussed many different ideas for incremental linking, but
>>> our conclusion is that you can only get one of Fast|Simple.  If you want it
>>> to be fast it has to be complicated and if you want it to be simple then
>>> it's going to be slow.
>>> Consider the case where you edit one .cpp file and change this:
>>> int x = 0, y = 7;
>>> to this:
>>> int x = 0;
>>> short y = 7;
>>> Because different instructions operate on shorts vs ints, some of the
>>> instruction encodings will be different and potentially of a different size.
>>> Because of this, the contribution to the .text section from this object
>>> file is going to be a different size.
>>> Because of that, all subsequent object files will start at a different
>>> absolute file address in the final executable.
>>> Because of that, every single symbol in every single object file will
>>> need to be updated in the final PDB.
>>> There are many other things that need to happen as well, but the point
>>> is that trivial change to a cpp file can explode into many changes in the
>>> final PDB.
>>> There are ways to handle this, but they're not simple.  We have some
>>> ideas, but for the moment we are focused on making full linking as fast as
>>> possible because it's much easier and still provides benefits.  We think we
>>> can get it fast enough that it will be acceptable, and that should give us
>>> some extra time to do incremental linking properly.
>>> On Mon, Jan 29, 2018 at 1:07 PM Leonardo Santagada <santagada at gmail.com>
>>> wrote:
>>>> About incremental linking, the only thing from my benchmark that needs
>>>> to be incremental is the pdb patching as generating the binary seems faster
>>>> than incremental linking on link.exe, so did anyone propose renaming the
>>>> current binary, writing a new one and then diffing the coff obj and using
>>>> that info to just rewriting that part of the pdb. Or another idea is making
>>>> the build system feed into the linker which files changed so the
>>>> types/debug information can be compared instead of all of them?
>>>> On Mon, Jan 29, 2018 at 7:55 PM, Zachary Turner <zturner at google.com>
>>>> wrote:
>>>>> Not a lot.
>>>>> /TIME will show high level timing of the various phases (this is the
>>>>> same option MSVC uses).
>>>>> If you want anything more detailed than that, vTune or ETW+WPA (
>>>>> https://github.com/google/UIforETW/releases) are probably what you'll
>>>>> need to do.
>>>>> (We'd definitely love patches to improve performance, or even just
>>>>> ideas about how to make things faster.  Improving link speed is one of our
>>>>> biggest priorities.)
>>>>> On Mon, Jan 29, 2018 at 10:47 AM Leonardo Santagada <
>>>>> santagada at gmail.com> wrote:
>>>>>> Yeah true, is there any switches to profile the linker?
>>>>>> On 29 Jan 2018 18:43, "Zachary Turner" <zturner at google.com> wrote:
>>>>>>> Part of the reason why lld is so fast is because we map every input
>>>>>>> file into memory up front and rely on the virtual memory manager in the
>>>>>>> kernel to make this fast.  Generally speaking, this is a lot faster than
>>>>>>> opening a file, reading it and processing a file, and closing the file.
>>>>>>> The downside, as you note, is that it uses a lot of memory.
>>>>>>> But there's a catch.  The kernel is smart enough to share the
>>>>>>> physical memory pages when you map the same file multiple times from
>>>>>>> multiple processes.  So it only looks like the memory usage is high because
>>>>>>> it reserves a large amount of address space in each process.  But the total
>>>>>>> amount of physical memory used will not increase when additional instances
>>>>>>> of the same file are mapped.
>>>>>>> On Mon, Jan 29, 2018 at 9:24 AM Leonardo Santagada <
>>>>>>> santagada at gmail.com> wrote:
>>>>>>>> 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
>>>> --
>>>> Leonardo Santagada


Leonardo Santagada
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