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

[Zachary Turner via llvm-dev]

Actually I already have a theory that even though you are adding the
section to the section table, you might not be adding a *symbol* for the
section to the symbol table.  So the existing symbols (which reference
sections by index) will all be wrong because you've inserted a new
section.  Still though, obj2yaml would expose that.

On Thu, Jan 25, 2018 at 9:50 AM Zachary Turner <zturner at google.com> wrote:

> Yea as long as you compare clang-cl object file with automatically
> generated .debug$H section against clang-cl object file without .debug$H
> but added after the fact with llvm-objcopy, that should expose the problem
> I think when you run obj2yaml on them.
>
> On Thu, Jan 25, 2018 at 9:49 AM Leonardo Santagada <santagada at gmail.com>
> wrote:
>
>> I did reorder my sections, so that .debug$H is in the correct place, but
>> now I get some errors on dubplicate symbols, I created a folder with
>> examples:
>>
>> https://www.dropbox.com/sh/nmvzi44pi0boe76/AAA0f47O5PCJ9JiUc6wVuwBra?dl=0
>>
>> t.obj is generated by vs 2015 and it links fine with lld-link.exe, but
>> tout.obj gives this errors:
>>
>> lld-link.exe /DEBUG:GHASH tout.obj
>> LLD-LINK.EXE: error: duplicate symbol: __local_stdio_printf_options in
>> tout.obj and in LIBCMT.lib(default_local_stdio_options.obj)
>> LLD-LINK.EXE: error: duplicate symbol: __local_stdio_printf_options in
>> tout.obj and in libvcruntime.lib(undname.obj)
>>
>> I'm using PEView from http://wjradburn.com/software/ to look at the
>> files and can't see anything wrong, except some valid differences in the
>> offsets being used for the data (so pointer to data is different between
>> them).
>>
>> I will look into yaml2obj now to see if I see anything else weird going
>> on.
>>
>>
>> On Thu, Jan 25, 2018 at 6:41 PM, Zachary Turner <zturner at google.com>
>> wrote:
>>
>>> I'm pretty confident that cl is not putting anything strange in the
>>> .debug$T sections.  We've done a lot of testing and never seen anything
>>> except CodeView type records in a .debug$T.  My hunch is that your objcopy
>>> patch is probably not doing the right thing in one or more of the section
>>> headers, and this is confusing the linker.
>>>
>>> One idea might be to build a simple object file with clang-cl but
>>> without the magic -mllvm -emit-codeview-ghash-section, then run your
>>> llvm-objcopy on it.  Then build the same object file passing -mllvm
>>> -emit-codeview-ghash-section.  Then run obj2yaml on both and diff the
>>> results.  They should be byte-for-byte identical.  That should give you a
>>> clue about if objcopy is doing something wrong.
>>>
>>> On Thu, Jan 25, 2018 at 2:21 AM Leonardo Santagada <santagada at gmail.com>
>>> wrote:
>>>
>>>> Don't worry, I definetly want to perfect this to generate legal obj
>>>> files, this is just to speed up testing.
>>>>
>>>> Now after patching all the obj files I get this errors when linking a
>>>> small part of our code base (msvc 2017 15.5.3, lld and llvm-objcopy 7.0.0):
>>>> lld-link.exe : error : relocation against symbol in discarded section:
>>>> $LN8
>>>> lld-link.exe : error : relocation against symbol in discarded section:
>>>> $LN43
>>>> lld-link.exe : error : relocation against symbol in discarded section:
>>>> $LN37
>>>>
>>>> I'm starting to guess that cl.exe might be putting some random comdat
>>>> or other discardable symbols in the .debug$T and clang doesn't? I will try
>>>> to debug this and see what more I can uncover.
>>>>
>>>> Linking works perfectly without my llvm-objcopy pass to add .debug$H?
>>>>
>>>>
>>>> On Thu, Jan 25, 2018 at 1:53 AM, Zachary Turner <zturner at google.com>
>>>> wrote:
>>>>
>>>>> It might not influence LLD, but at the same time we don't want to
>>>>> upstream something that is producing technically illegal COFF files.  Also
>>>>> good to hear about the planned changes to your header files.  Looking
>>>>> forward to hearing about your experiences with clang-cl.
>>>>>
>>>>> On Wed, Jan 24, 2018 at 10:41 AM Leonardo Santagada <
>>>>> santagada at gmail.com> wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> I finally got my first .obj file patched with .debug$H to look
>>>>>> somewhat right. I added the new section at the end of the file so I don't
>>>>>> have to recalculate all sections (although now I probably could position it
>>>>>> in the middle, knowing that each section is: SizeOfRawData +
>>>>>> (last.Header.NumberOfRelocations * (4+4+2)) and the $H needs to come right
>>>>>> after $T in the file). That although illegal based on the coff specs
>>>>>> doesn't seem its going to influence lld.
>>>>>>
>>>>>> Also we talked and we are probably going to do something similar to a
>>>>>> bunch of windows defines and a check for our own define (to guarantee that
>>>>>> no one imported windows.h before win32.h) and drop the namespace and the
>>>>>> conflicting names.
>>>>>>
>>>>>>
>>>>>> On Tue, Jan 23, 2018 at 12:46 AM, Zachary Turner <zturner at google.com>
>>>>>> wrote:
>>>>>>
>>>>>>> That's very possible that a 3rd party indirect header include is
>>>>>>> involved.  One idea might be like I suggested where you #define _WINDOWS_
>>>>>>> in win32.h and guarantee that it's always included first.  Then those other
>>>>>>> headers won't be able to #include <windows.h>.  but it will probably
>>>>>>> greatly expand the amount of stuff you have to add to win32.h, as you will
>>>>>>> probably find some callers of functions that aren't yet in your win32.h
>>>>>>> that you'd have to add.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Mon, Jan 22, 2018 at 3:28 PM Leonardo Santagada <
>>>>>>> santagada at gmail.com> wrote:
>>>>>>>
>>>>>>>> Ok some information was lost on getting this example to you, I'm
>>>>>>>> sorry for not being clear.
>>>>>>>>
>>>>>>>> We have a huge code base, let's say 90% of it doesn't include
>>>>>>>> either header, 9% include win32.h and 1% includes both, I will try to
>>>>>>>> discover why, but my guess is they include both a third party that includes
>>>>>>>> windows.h and some of our libs that use win32.h.
>>>>>>>>
>>>>>>>> I will try to fully understand this tomorrow.
>>>>>>>>
>>>>>>>> I guess clang will not implement this ever so finishing the object
>>>>>>>> copier is the best solution until all code is ported to clang.
>>>>>>>>
>>>>>>>> On 23 Jan 2018 00:02, "Zachary Turner" <zturner at google.com> wrote:
>>>>>>>>
>>>>>>>>> You said win32.h doesn't include windows.h, but main.cpp does.  So
>>>>>>>>> what's the disadvantage of just including it in win32.h anyway, since it's
>>>>>>>>> already going to be in every translation unit?  (Unless you didn't mean to
>>>>>>>>> #include it in main.cpp)
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I guess all I can do is warn you how bad of an idea this is.  For
>>>>>>>>> starters, I already found a bug in your code ;-)
>>>>>>>>>
>>>>>>>>> // stdint.h
>>>>>>>>> typedef int                int32_t;
>>>>>>>>>
>>>>>>>>> // winnt.h
>>>>>>>>> typedef long LONG;
>>>>>>>>>
>>>>>>>>> // windef.h
>>>>>>>>> typedef struct tagPOINT
>>>>>>>>> {
>>>>>>>>>     LONG  x;   // long x
>>>>>>>>>     LONG  y;   // long y
>>>>>>>>> } POINT, *PPOINT, NEAR *NPPOINT, FAR *LPPOINT;
>>>>>>>>>
>>>>>>>>> // win32.h
>>>>>>>>> typedef int32_t LONG;
>>>>>>>>>
>>>>>>>>> struct POINT
>>>>>>>>> {
>>>>>>>>> LONG x;   // int x
>>>>>>>>> LONG y;   // int y
>>>>>>>>> };
>>>>>>>>>
>>>>>>>>> So POINT is defined two different ways.  In your minimal
>>>>>>>>> interface, it's declared as 2 int32's, which are int.  In the actual
>>>>>>>>> Windows header files, it's declared as 2 longs.
>>>>>>>>>
>>>>>>>>> This might seem like a unimportant bug since int and long are the
>>>>>>>>> same size, but int and long also mangle differently and affect overload
>>>>>>>>> resolution, so you could have weird linker errors or call the wrong
>>>>>>>>> function overload.
>>>>>>>>>
>>>>>>>>> Plus, it illustrates the fact that this struct *actually is* a
>>>>>>>>> different type from the one in the windows header.
>>>>>>>>>
>>>>>>>>> You said at the end that you never intentionally import win32.h
>>>>>>>>> and windows.h from the same translation unit.  But then in this example you
>>>>>>>>> did.  I wonder if you could enforce that by doing this:
>>>>>>>>>
>>>>>>>>> // win32.h
>>>>>>>>> #pragma once
>>>>>>>>>
>>>>>>>>> // Error if windows.h was included before us.
>>>>>>>>> #if defined(_WINDOWS_)
>>>>>>>>> #error "You're including win32.h after having already included
>>>>>>>>> windows.h.  Don't do this!"
>>>>>>>>> #endif
>>>>>>>>>
>>>>>>>>> // And also make sure windows.h can't get included after us
>>>>>>>>> #define _WINDOWS_
>>>>>>>>>
>>>>>>>>> For the record, I tried the test case you linked when windows.h is
>>>>>>>>> not included in main.cpp and it works (but still has the bug about int and
>>>>>>>>> long).
>>>>>>>>>
>>>>>>>>> On Mon, Jan 22, 2018 at 2:23 PM Leonardo Santagada <
>>>>>>>>> santagada at gmail.com> wrote:
>>>>>>>>>
>>>>>>>>>> It is super gross, but we copy parts of windows.h because having
>>>>>>>>>> all of it if both gigantic and very very messy. So our win32.h has a couple
>>>>>>>>>> thousands of lines and not 30k+ for windows.h and we try to have zero
>>>>>>>>>> macros. Win32.h doesn't include windows.h so using ::BOOL wouldn't work. We
>>>>>>>>>> don't want to create a namespace, we just want a cleaner interface to
>>>>>>>>>> windows api. The namespace with c linkage is the way to trick cl into
>>>>>>>>>> allowing us to in some files have both windows.h and Win32.h. I really
>>>>>>>>>> don't see any way for us to have this Win32.h without this cl support, so
>>>>>>>>>> maybe we should either put windows.h in a compiled header somewhere and not
>>>>>>>>>> care that it is infecting everything or just have one place we can call to
>>>>>>>>>> clean up after including windows.h (a massive set of undefs).
>>>>>>>>>>
>>>>>>>>>> So using can't work, because we never intentionally import
>>>>>>>>>> windows.h and win32.h on the same translation unit.
>>>>>>>>>>
>>>>>>>>>> On Mon, Jan 22, 2018 at 7:08 PM, Zachary Turner <
>>>>>>>>>> zturner at google.com> wrote:
>>>>>>>>>>
>>>>>>>>>>> This is pretty gross, honestly :)
>>>>>>>>>>>
>>>>>>>>>>> Can't you just use using declarations?
>>>>>>>>>>>
>>>>>>>>>>> namespace Win32 {
>>>>>>>>>>> extern "C" {
>>>>>>>>>>>
>>>>>>>>>>> using ::BOOL;
>>>>>>>>>>> using ::LONG;
>>>>>>>>>>> using ::POINT;
>>>>>>>>>>> using ::LPPOINT;
>>>>>>>>>>>
>>>>>>>>>>> using ::GetCursorPos;
>>>>>>>>>>> }
>>>>>>>>>>> }
>>>>>>>>>>>
>>>>>>>>>>> This works with clang-cl.
>>>>>>>>>>>
>>>>>>>>>>> On Mon, Jan 22, 2018 at 5:39 AM Leonardo Santagada <
>>>>>>>>>>> santagada at gmail.com> wrote:
>>>>>>>>>>>
>>>>>>>>>>>> Here it is a minimal example, we do this so we don't have to
>>>>>>>>>>>> import the whole windows api everywhere.
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> https://gist.github.com/santagada/7977e929d31c629c4bf18ebb987f6be3
>>>>>>>>>>>>
>>>>>>>>>>>> On Sun, Jan 21, 2018 at 2:31 AM, Zachary Turner <
>>>>>>>>>>>> zturner at google.com> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> Clang-cl maintains compatibility with msvc even in cases where
>>>>>>>>>>>>> it’s non standards compliant (eg 2 phase name lookup), but we try to keep
>>>>>>>>>>>>> these cases few and far between.
>>>>>>>>>>>>>
>>>>>>>>>>>>> To help me understand your case, do you mean you copy
>>>>>>>>>>>>> windows.h and modify it? How does this lead to the same struct being
>>>>>>>>>>>>> defined twice? If i were to write this:
>>>>>>>>>>>>>
>>>>>>>>>>>>> struct Foo {};
>>>>>>>>>>>>> struct Foo {};
>>>>>>>>>>>>>
>>>>>>>>>>>>> Is this a small repro of the issue you’re talking about?
>>>>>>>>>>>>>
>>>>>>>>>>>>> On Sat, Jan 20, 2018 at 3:44 PM Leonardo Santagada <
>>>>>>>>>>>>> santagada at gmail.com> wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>>> I can totally see something like incremental linking with a
>>>>>>>>>>>>>> simple padding between obj and a mapping file (which can also help with
>>>>>>>>>>>>>> edit and continue, something we also would love to have).
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> We have another developer doing the port to support clang-cl,
>>>>>>>>>>>>>> but although most of our code also goes trough a version of clang,
>>>>>>>>>>>>>> migrating the rest to clang-cl has been a fight. From what I heard the main
>>>>>>>>>>>>>> problem is that we have a copy of parts of windows.h (so not to bring the
>>>>>>>>>>>>>> awful parts of it like lower case macros) and that totally works on cl, but
>>>>>>>>>>>>>> clang (at least 6.0) complains about two struct/vars with the same name,
>>>>>>>>>>>>>> even though they are exactly the same. Making clang-cl as broken as cl.exe
>>>>>>>>>>>>>> is not an option I suppose? I would love to turn on a flag
>>>>>>>>>>>>>> --accept-that-cl-made-bad-decisions-and-live-with-it and have this at least
>>>>>>>>>>>>>> until this is completely fixed in our code base.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> the biggest win with moving to cl would be a better more
>>>>>>>>>>>>>> standards compliant compiler, no 1 minute compiles on heavily templated
>>>>>>>>>>>>>> files and maybe the holy grail of ThinLTO.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> On Sat, Jan 20, 2018 at 10:56 PM, Zachary Turner <
>>>>>>>>>>>>>> zturner at google.com> wrote:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> 10-15s will be hard without true incremental linking.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> At some point that's going to be the only way to get any
>>>>>>>>>>>>>>> faster, but incremental linking is hard (putting it lightly), and since our
>>>>>>>>>>>>>>> full links are already really fast we think we can get reasonably close to
>>>>>>>>>>>>>>> link.exe incremental speeds with full links.  But it's never enough and I
>>>>>>>>>>>>>>> will always want it to be faster, so you may see incremental linking in the
>>>>>>>>>>>>>>> future after we hit a performance wall with full link speed :)
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> In any case, I'm definitely interested in seeing what kind
>>>>>>>>>>>>>>> of numbers you get with /debug:ghash after you get this llvm-objcopy
>>>>>>>>>>>>>>> feature implemented.  So keep me updated :)
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> As an aside, have you tried building with clang instead of
>>>>>>>>>>>>>>> cl?  If you build with clang you wouldn't even have to do this llvm-objcopy
>>>>>>>>>>>>>>> work, because it would "just work".  If you've tried but ran into issues
>>>>>>>>>>>>>>> I'm interested in hearing about those too.  On the other hand, it's also
>>>>>>>>>>>>>>> reasonable to only switch one thing at a time.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> On Sat, Jan 20, 2018 at 1:34 PM Leonardo Santagada <
>>>>>>>>>>>>>>> santagada at gmail.com> wrote:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> if we get to < 30s I think most users would prefer it to
>>>>>>>>>>>>>>>> link.exe, just hopping there is still some more optimizations to get closer
>>>>>>>>>>>>>>>> to ELF linking times (around 10-15s here).
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> On Sat, Jan 20, 2018 at 9:50 PM, Zachary Turner <
>>>>>>>>>>>>>>>> zturner at google.com> wrote:
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Generally speaking a good rule of thumb is that
>>>>>>>>>>>>>>>>> /debug:ghash will be close to or faster than /debug:fastlink, but with none
>>>>>>>>>>>>>>>>> of the penalties like slow debug time
>>>>>>>>>>>>>>>>> On Sat, Jan 20, 2018 at 12:44 PM Zachary Turner <
>>>>>>>>>>>>>>>>> zturner at google.com> wrote:
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> 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
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> --
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Leonardo Santagada
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> --
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Leonardo Santagada
>>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> --
>>>>>>>>>>>>
>>>>>>>>>>>> Leonardo Santagada
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> --
>>>>>>>>>>
>>>>>>>>>> Leonardo Santagada
>>>>>>>>>>
>>>>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>>
>>>>>> Leonardo Santagada
>>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>>
>>>> Leonardo Santagada
>>>>
>>>
>>
>>
>> --
>>
>> Leonardo Santagada
>>
>
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