[llvm-dev] Building libjpeg-turbo with LTO

[Teresa Johnson via llvm-dev]

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. 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

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 |
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