llvm dev - Dec 2013 - [LLVMdev] Unable to perform LLVM LTO using clang

Hi everyone,

I want to perform whole-program analysis for apache httpd using httpd's
original autoconf system so that I need to generate a single bitcode file.

So I follow the instructions in LLVM document on gold plugin
http://llvm.org/docs/GoldPlugin.html#lto-how-to-build. However, when
running a simple program using LTO, clang fails to link the object file
with the LLVM bitcode.
There're only two c files, a.c and b.c in the source folder.

$ clang -flto a.c -c -o a.o      # <-- a.o is LLVM bitcode file$ clang
b.c -c -o b.o            # <-- b.o is native object file$ clang -flto
a.o b.o -o main    # <-- link with LLVMgold plugin

When executing the last command, it prompts
> a.o: file not recognized: File format not recognized
> clang: error: linker command failed with exit code 1 (use -v to see
> invocation)
At the same time, performing the command of
ld -plugin=/usr/local/lib/LLVMgold.so a.o b.o -o main
it will successfully generate main.

My libLTO build process is as follows,

   1. check out binutils
   2. configure with --enable-gold --enable-plugins
   3. make
   4. make install
   5. replace original ld in /usr/local/bin with ld.gold using ln
   6. replace ar and nm in /usr/local/bin with ar and nm-new in the
   binutils folder just built using ln
   7. configure LLVM
   with --with-binutils-include=/path/to/binutils/src/include
   8. execute make again in LLVM root

It seems the gold linker works well with the LLVMgold.so lib however clang
fails to correctly invoke gold linker.
Could anyone tell me where my mistake is?

Regards,
Chunbai Yang
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> Could anyone tell me where my mistake is?
Try running

clang -flto a.o b.o -o main -v

it should print how clang is running the linker. See if it is running
the linker you expect and if it is passing -plugin to it.

Cheers,
Rafael

Dear Rafael,

Thank you for your answer. The verbose mode -v shows that clang uses the
old ld in /usr/bin while I install the gold in /usr/local/bin.

Regards,
Chunbai


On Tue, Dec 3, 2013 at 9:23 PM, Rafael Espíndola <rafael.espindola at
gmail.com
> wrote:
> > Could anyone tell me where my mistake is?
>
> Try running
>
> clang -flto a.o b.o -o main -v
>
> it should print how clang is running the linker. See if it is running
> the linker you expect and if it is passing -plugin to it.
>
> Cheers,
> Rafael
>
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