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

Thu Jan 25 09:57:27 PST 2018

Any idea on how to create this new symbol there? I saw that there is a
symbol pointing to each section, but didn't understand the format, and
yaml2obj doesn't check it or do anything with the list.

On Thu, Jan 25, 2018 at 6:56 PM, Leonardo Santagada <santagada at gmail.com>
wrote:

> YES, THANK YOU... I WAS THINKING THIS BUT COMPLETELY FORGOT.
>
> sorry for the caps... long day of working on this, and using vs 2017,
> which adds a new section type .chks64 that I couldn't find documentation
> anywhere was difficult. I highly recommend everyone to just not using vs
> 2017 until 15.8 or something, our internal bug list is gigantic.
>
> On Thu, Jan 25, 2018 at 6:52 PM, Zachary Turner <zturner at google.com>
> wrote:
>
>> 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$H is in the correct place,
>>>> but now I get some errors on dubplicate symbols, I created a folder with
>>>> examples:
>>>>
>>>> https://www.dropbox.com/sh/nmvzi44pi0boe76/AAA0f47O5PCJ9JiUc
>>>> 6wVuwBra?dl=0
>>>>
>>>> t.obj is generated by vs 2015 and it links fine with lld-link.exe, but
>>>> tout.obj gives this errors:
>>>>
>>>> lld-link.exe /DEBUG:GHASH tout.obj
>>>> LLD-LINK.EXE: error: duplicate symbol: __local_stdio_printf_options in
>>>> tout.obj and in LIBCMT.lib(default_local_stdio_options.obj)
>>>> LLD-LINK.EXE: error: duplicate symbol: __local_stdio_printf_options in
>>>> tout.obj and in libvcruntime.lib(undname.obj)
>>>>
>>>> I'm using PEView from http://wjradburn.com/software/ to look at the
>>>> files and can't see anything wrong, except some valid differences in the
>>>> offsets being used for the data (so pointer to data is different between
>>>> them).
>>>>
>>>> I will look into yaml2obj now to see if I see anything else weird going
>>>> on.
>>>>
>>>>
>>>> On Thu, Jan 25, 2018 at 6:41 PM, Zachary Turner <zturner at google.com>
>>>> wrote:
>>>>
>>>>> I'm pretty confident that cl is not putting anything strange in the
>>>>> .debug$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/7977e929d31c629c4bf18ebb98
>>>>>>>>>>>>>> 7f6be3
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> 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
>>>>
>>>
>
>
> --
>
> Leonardo Santagada
>

-- 

Leonardo Santagada
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