[LLVMdev] libLTO on linux

[Chandler Carruth]

On Sat, Nov 29, 2008 at 9:17 PM, Nick Lewycky <nicholas at mxc.ca> wrote:
> I'm looking into the possibility of building the libLTO plugin on Linux,
> and got tangled up in the build system.
>
> Currently, tools/Makefile only includes 'lto' when the OS equals Darwin,
> while tools/lto/Makefile has an if-statement that builds libLTO as a
> static library on non-Darwin systems.
>
> If I try to build libLTO as a dynamic library on Linux, it works fine,
> but I get an enormous warning from libtool:
> *** Warning: Linking the shared library
> /home/nicholas/llvm-commit/Debug/lib/libLTO.la against the non-libtool
> *** objects  /home/nicholas/llvm-commit/Debug/lib/LLVMCppBackend.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMMSIL.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMCBackend.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMXCore.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMPIC16.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMCellSPUCodeGen.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMCellSPUAsmPrinter.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMMips.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMARMAsmPrinter.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMARMCodeGen.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMIA64.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMAlphaCodeGen.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMAlphaAsmPrinter.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMPowerPCAsmPrinter.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMPowerPCCodeGen.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMSparcCodeGen.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMSparcAsmPrinter.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMX86AsmPrinter.o
> /home/nicholas/llvm-commit/Debug/lib/LLVMX86CodeGen.o is not portable!
>
> If we're going to build a .so file, we need those .o's to be built with
> -fPIC. As far as I can see, configuring LLVM with --enable-pic actually
> does that already, so perhaps we can make it the default.
>
> But that warning isn't about PIC, it's about the fact that the .o's
> weren't compiled with libtool. It's looking for the matching .lo files.

My question is this, why use libtool to build the .so? I don't see
what libtool helps with here, and it is a nightmare to use in general.
Why not directly use the gcc driver?

-Chandler

>
> It's not clear to me what we ought to do here. Put 'lto' inside an if
> ENABLE_PIC and suppress the warning? Build everything in LLVM with
> libtool --mode=compile? Is there a simple way to limit that to just the
> .o files we end up installing?
>
> Nick
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