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

Sat Jan 20 12:44:41 PST 2018

Chrome is actually one of my exact benchmark cases. When building
blink_core.dll and browser_tests.exe, i get anywhere from a 20-40%
reduction in link time. We have some other optimizations in the pipeline
but not upstream yet.

My best time so far (including other optimizations not yet upstream) is 28s
on blink_core.dll, compared to 110s with /debug
On Sat, Jan 20, 2018 at 12:28 PM Leonardo Santagada <santagada at gmail.com>
wrote:

> On Sat, Jan 20, 2018 at 9:05 PM, Zachary Turner <zturner at google.com>
> wrote:
>
>> You probably don't want to go down the same route that clang goes through
>> to write the object file.  If you think yaml2coff is convoluted, the way
>> clang does it will just give you a headache.  There are multiple
>> abstractions involved to account for different object file formats (ELF,
>> COFF, MachO) and output formats (Assembly, binary file).  At least with
>> yaml2coff
>>
>
> I think your phrase got cut there, but yeah I just found AsmPrinter.cpp
> and it is convoluted.
>
>
>
>> It's true that yaml2coff is using the COFFParser structure, but if you
>> look at the writeCOFF function in yaml2coff it's pretty bare-metal.  The
>> logic you need will be almost identical, except that instead of checking
>> the COFFParser for the various fields, you'll check the existing
>> COFFObjectFile, which should have similar fields.
>>
>> The only thing you need to different is when writing the section table
>> and section contents, to insert a new entry.  Since you're injecting a
>> section into the middle, you'll also probably need to push back the file
>> pointer of all subsequent sections so that they don't overlap.  (e.g. if
>> the original sections are 1, 2, 3, 4, 5 and you insert between 2 and 3,
>> then the original sections 3, 4, and 5 would need to have their
>> FilePointerToRawData offset by the size of the new section).
>>
>
> I have the PE/COFF spec open here and I'm happy that I read a bit of it so
> I actually know what you are talking about... yeah it doesn't seem too
> complicated.
>
>
>
>> If you need to know what values to put for the other fields in a section
>> header, run `dumpbin /headers foo.obj` on a clang-generated object file
>> that has a .debug$H section already (e.g. run clang with
>> -emit-codeview-ghash-section, and look at the properties of the .debug$H
>> section and use the same values).
>>
>
> Thanks I will do that and then also look at how the CodeView part of the
> code does it if I can't understand some of it.
>
>
>> The only invariant that needs to be maintained is that
>> Section[N]->FilePointerOfRawData == Section[N-1]->FilePointerOfRawData +
>> Section[N-1]->SizeOfRawData
>>
>
> Well, that and all the sections need to be on the final file... But I'm
> hopeful.
>
>
> Anyone has times on linking a big project like chrome with this so that at
> least I know what kind of performance to expect?
>
> My numbers are something like:
>
> 1 pdb per obj file: link.exe takes ~15 minutes and 16GB of ram,
> lld-link.exe takes 2:30 minutes and ~8GB of ram
> around 10 pdbs per folder: link.exe takes 1 minute and 2-3GB of ram,
> lld-link.exe takes 1:30 minutes and ~6GB of ram
> faslink: link.exe takes 40 seconds, but then 20 seconds of loading at the
> first break point in the debugger and we lost DIA support for listing
> symbols.
> incremental: link.exe takes 8 seconds, but it only happens when very minor
> changes happen.
>
> We have an non negligible number of symbols used on some runtime systems.
>
>
>>
>> On Sat, Jan 20, 2018 at 11:52 AM Leonardo Santagada <santagada at gmail.com>
>> wrote:
>>
>>> Thanks for the tips, I now have something that reads the obj file, finds
>>> .debug$T sections and global hashes it (proof of concept kind of code).
>>> What I can't find is: how does clang itself writes the coff files with
>>> global hashes, as that might help me understand how to create the .debug$H
>>> section, how to update the file section count and how to properly write
>>> this back.
>>>
>>> The code on yaml2coff is expecting to be working on the yaml COFFParser
>>> struct and I'm having quite a bit of a headache turning the COFFObjectFile
>>> into a COFFParser object or compatible... Tomorrow I might try the very non
>>> efficient path of coff2yaml and then yaml2coff with the hashes header...
>>> but it seems way too inefficient and convoluted.
>>>
>>> On Fri, Jan 19, 2018 at 10:38 PM, Zachary Turner <zturner at google.com>
>>> wrote:
>>>
>>>>
>>>>
>>>> On Fri, Jan 19, 2018 at 1:02 PM Leonardo Santagada <santagada at gmail.com>
>>>> wrote:
>>>>
>>>>> On Fri, Jan 19, 2018 at 9:44 PM, Zachary Turner <zturner at google.com>
>>>>> wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>> On Fri, Jan 19, 2018 at 12:29 PM Leonardo Santagada <
>>>>>> santagada at gmail.com> wrote:
>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>> No I didn't, I used cl.exe from the visual studio toolchain. What
>>>>>>> I'm proposing is a tool for processing .obj files in COFF format, reading
>>>>>>> them and generating the GHASH part.
>>>>>>>
>>>>>>> To make our build faster we use hundreds of unity build files
>>>>>>> (.cpp's with a lot of other .cpp's in them aka munch files) but still have
>>>>>>> a lot of single .cpp's as well (in total something like 3.4k .obj files).
>>>>>>>
>>>>>>> ps: sorry for sending to the wrong list, I was reading about llvm
>>>>>>> mailing lists and jumped when I saw what I thought was a lld exclusive list.
>>>>>>>
>>>>>>
>>>>>> A tool like this would be useful, yes.  We've talked about it
>>>>>> internally as well and agreed it would be useful, we just haven't
>>>>>> prioritized it.  If you're interested in submitting a patch along those
>>>>>> lines though, I think it would be a good addition.
>>>>>>
>>>>>> I'm not sure what the best place for it would be.  llvm-readobj and
>>>>>> llvm-objdump seem like obvious choices, but they are intended to be
>>>>>> read-only, so perhaps they wouldn't be a good fit.
>>>>>>
>>>>>> llvm-pdbutil is kind of a hodgepodge of everything else related to
>>>>>> PDBs and symbols, so I wouldn't be opposed to making a new subcommand there
>>>>>> called "ghash" or something that could process an object file and output a
>>>>>> new object file with a .debug$H section.
>>>>>>
>>>>>> A third option would be to make a new tool for it.
>>>>>>
>>>>>> I don't htink it would be that hard to write.  If you're interested
>>>>>> in trying to make a patch for this, I can offer some guidance on where to
>>>>>> look in the code.  Otherwise it's something that we'll probably get to, I'm
>>>>>> just not sure when.
>>>>>>
>>>>>>>
>>>>> I would love to write it and contribute it back, please do tell, I did
>>>>> find some of the code of ghash in lld, but in fuzzy on the llvm codeview
>>>>> part of it and never seen llvm-readobj/objdump or llvm-pdbutil, but I'm not
>>>>> afraid to look :)
>>>>>
>>>>>
>>>>  Luckily all of the important code is hidden behind library calls, and
>>>> it should already just do the right thing, so I suspect you won't need to
>>>> know much about CodeView to do this.
>>>>
>>>> I think Peter has the right idea about putting this in llvm-objcopy.
>>>>
>>>> You can look at one of the existing CopyBinary functions there, which
>>>> currently only work for ELF, but you can just make a new overload that
>>>> accepts a COFFObjectFile.
>>>>
>>>> I would probably start by iterating over each of the sections
>>>> (getNumberOfSections / getSectionName) looking for .debug$T and .debug$H
>>>> sections.
>>>>
>>>> If you find a .debug$H section then you can just skip that object
>>>> file.
>>>>
>>>> If you find a .debug$T but not a .debug$H, then basically do the same
>>>> thing that LLD does in PDBLinker::mergeDebugT  (create a CVTypeArray, and
>>>> pass it to GloballyHashedType::hashTypes.  That will return an array of
>>>> hash values.  (the format of .debug$H is the header, followed by the hash
>>>> values).  Then when you're writing the list of sections, just add in the
>>>> .debug$H section right after the .debug$T section.
>>>>
>>>> Currently llvm-objcopy only writes ELF files, so it would need to be
>>>> taught to write COFF files.  We have code to do this in the yaml2obj
>>>> utility (specifically, in yaml2coff.cpp in the function writeCOFF).  There
>>>> may be a way to move this code to somewhere else (llvm/Object/COFF.h?) so
>>>> that it can be re-used by both yaml2coff and llvm-objcopy, but in the worst
>>>> case scenario you could copy the code and re-write it to work with these
>>>> new structures.
>>>>
>>>> Lastly, you'll probably want to put all of this behind an option in
>>>> llvm-objcopy such as -add-codeview-ghash-section
>>>>
>>>>
>>>
>>>
>>> --
>>>
>>> Leonardo Santagada
>>>
>>
>
>
> --
>
> Leonardo Santagada
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180120/983ee363/attachment.html>