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

[Zachary Turner via llvm-dev]

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
>
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