Clang-3.8 and compiler-rt vs libgcc

Question

I used clang-3.5 to happily create musl libc bit code versions and use the result to create great standalone executables.

Recent attempts with clang-3.8 not been so happy. The clang-3.8 bitcode seems to generate the functions defined in

 compiler-rt/lib/builtins 

Typical examples of functions that I consider polluting the bit code are mulxc3 , mulsc3 and muldc3 . I can solve this by contacting libgcc or even the llvm alternative if I had a clear idea of ​​what it was. Although I would prefer to prevent the occurrence of the problem in the first place.

I saw mention of flags like rtlib=compiler-rt , etc., but found very little documentation on this.

So here are some simple questions.

• Is it possible to prevent the use of clang compiler-rt/lib/builtins in the emitted bit code? Or if not

• Does llvm create a version of libgcc that I could use. In fact, I would probably build its version with bit code, but this does not apply to this topic.

Love to hear some recommendations about this.

Added on 12/8/2016:. Therefore, I will illustrate my problems with a specific workflow that people can reproduce if they wish, or, more likely, just indicate where I am stupid.

So, start by checking:

musllv

and follow the instructions in README.to to compile (here I use clang-3.8 on ubuntu 14.04)

 WLLVM_CONFIGURE_ONLY=1 CC=wllvm ./configure --target=LLVM --build=LLVM make cd lib extract-bc -b libc.a 

You will also need the bit code of a simple executable. I will use nweb.c here.

  wllvm nweb.c -o nweb extract-bc nweb 

Now we can do things like:

 clang -static -nostdlib nweb.bc libc.a.bc crt1.o libc.a -o nweb 

This workflow runs smoothly for clang-3.5, but for clang-3.8 we get:

 clang -static -nostdlib nweb.bc libc.a.bc crt1.o libc.a -o nweb /tmp/libc-f734a3.o: In function `cpowl': libc.a.bc:(.text+0xbb9a): undefined reference to `__mulxc3' /tmp/libc-f734a3.o: In function `cpowf': libc.a.bc:(.text+0x38f7d): undefined reference to `__mulsc3' /tmp/libc-f734a3.o: In function `csqrt': libc.a.bc:(.text+0x78fc3): undefined reference to `__muldc3' /tmp/libc-f734a3.o: In function `cpow': libc.a.bc:(.text+0xafafc): undefined reference to `__muldc3' clang-3.8: error: linker command failed with exit code 1 (use -v to seeinvocation) 

Since @ paul-brannan indicates that we could try

 clang -static -nostdlib --rtlib=compiler-rt nweb.bc libc.a.bc crt1.o libc.a -o nweb 

But here is where I am probably stupid, because I get:

 clang-3.8: warning: argument unused during compilation: '--rtlib=compiler-rt' 

regardless of whether I use it as a binding or compilation flag.

Answer 1

OK, so I finally managed to achieve this. I built llvm-3.8.1 along with the compiler-rt project using wllvm and wllvm++ .

One of the build products was libclang_rt.builtins-x86_64.a , and from this archive I was able to extract the bit code module

libclang_rt.builtins-x86_64.bc

using the command: extract-bc -b libclang_rt.builtins-x86_64.a This bit code module has definitions for those annoying instrinsics like __mulxc3 , __mulsc3 and __muldc3 .

Hallelujah!