[llvm-dev] Building libjpeg-turbo with LTO

[Fangrui Song via llvm-dev]

On 2020-04-08, Teresa Johnson wrote:
>Adding a couple of lld folks.
>
>I helped Shishir debug this, the link line looked like:
>   /home/sjessu/build/bin/clang -O0 -flto -o jcstest jcstest.o
> ./.libs/libjpeg.a
>and the issue was that libjpeg.a was created with the system ar instead of
>llvm-ar. It worked when recreating libjpeg.a with llvm-ar.
>
>I noticed that the lld code has some special handling for the case
>when there is a missing symbol table, which often happens with system ar
>created archives containing bitcode. I noticed that the lld code will
>sometimes emit an error, but actually contains a special hack to handle
>archives containing *only* bitcode objects, so that they are handled
>correctly even when there is no symbol table because it was created with
>the system ar.

https://reviews.llvm.org/D63781 added the "archive has no index; run
ranlib to add one" error.

A clarification: the LinkerDriver::addFile code handles mix-and-match
ELF object members and bitcode members. A LazyObjFile can be
either an ELF object file or an LLVM bitcode file.

>Unfortunately, in this case it neither gave an error nor did
>the special handling, because libjpeg.a also contains some native objects
>and thus had a non-zero symbol table. I created a version of libjpeg.a
>using the system library and containing only the bitcode objects, and
>confirmed it links fine with lld (the native objects weren't needed in this
>case). BTW this is the code in ELF/Driver.cpp LinkerDriver::addFile.
>
>Would it be possible to extend the hack in lld to handle cases like this
>with some bitcode objects and some non-bitcode objects, so that the bitcode
>objects are not simply ignored?
>
>Thanks,
>Teresa

I guess what happened here is that the archive has an incomplete symbol table.
nm -s (--print-armap) can print the symbol table.

   % ar rc a.a a.bc a.o; nm -s a.a
   
   Archive index:
   _start in a.o
   nm: a.bc: file format not recognized
   
   a.o:
   0000000000000000 T _start

Currently lld trusts the archive symbol table. If the archive symbol table
actually misses some entries (GNU ar does not add bitcode definitions to the
symbol table), lld will not know that some lazy definitions are actually
missing.

It seems that if we have to make the GNU ar scenario work, lld has to distrust the archive symbol table when it contains bitcode files...
To not pessimize the case with all bitcode members but no ELF object members, we need to refine the hack to "distrust" the archive symbol table
if (the archive symbol table exists && an ELF object member exists && a bitcode member exists).

Does this scheme sound good?

>On Wed, Apr 8, 2020 at 10:25 AM Shishir V Jessu via llvm-dev <
>llvm-dev at lists.llvm.org> wrote:
>
>> To correct a typo: I am using both clang 6.0.0, and a local build of clang
>> 10.0.0, and each result in the same error.
>>
>> Best,
>> Shishir Jessu
>>
>> On Wed, Apr 8, 2020 at 12:22 PM Shishir V Jessu <shishir.jessu at utexas.edu>
>> wrote:
>>
>>> Hi,
>>>
>>> I have tried to build libjpeg-turbo
>>> <https://github.com/libjpeg-turbo/libjpeg-turbo> with LTO in LLVM, using
>>> both clangbut get many errors in lld that look like the following:
>>>
>>> ld: error: undefined symbol: jpeg_std_error
>>> >>> referenced by jcstest.c:76
>>> >>>               lto.tmp:(main)
>>>
>>> ld: error: undefined symbol: jpeg_CreateCompress
>>> >>> referenced by jcstest.c:86
>>> >>>               lto.tmp:(main)
>>>
>>> ld: error: undefined symbol: jpeg_set_defaults
>>> >>> referenced by jcstest.c:88
>>> >>>               lto.tmp:(main)
>>>
>>> ld: error: undefined symbol: jpeg_default_colorspace
>>> >>> referenced by jcstest.c:90
>>> >>>               lto.tmp:(main)
>>> >>> referenced by jcstest.c:114
>>> >>>               lto.tmp:(main)
>>>
>>> This only occurs when compiling with the -flto flag. Has anyone been able
>>> to build libjpeg-turbo with LTO? Are there any modifications I need to make
>>> to the makefile or other configuration in order to do so? Thanks for your
>>> help!
>>>
>>> Best,
>>> Shishir Jessu
>>>
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>>
>
>
>-- 
>Teresa Johnson |  Software Engineer |  tejohnson at google.com |