[llvm-dev] convert a compile-time pass to an lto pass

[Mehdi AMINI via llvm-dev]

On Mon, Jul 22, 2019 at 8:41 PM Heiko via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> Hi,
>
> I am trying to convert a compile-time legacy module
> pass (so) to a link-time pass. I am using ArchLinux amd64
> with LLVM 8.0.0 but looking for a way to support llvm 3.8 up to current.
>
> To keep things simple, i am just counting modules, functions, basic blocks.
> For compile time usage I used 'clang -Xclang -load -Xclang mypass.so ...'.
> That is working as expected.
>
> In llvm/Transforms/IPO/PassManagerBuilder.h I looked for a suitable value
> in enum ExtensionPointTy, but found nothing for running at linking time.
>

One has been added a few weeks ago I believe.



>
> Next I tried to use
> clang -flto -fuse-ld=gold -Xclang -load -Xclang mypass.so ...'
> but then the pass is also run once per translation unit not once at link
> time.
>
> Since the linker does not know '-load', I am now using
> clang -flto -fuse-ld=gold -Xlinker -plugin -Xlinker mypass.so ...
> with the effect that the linker cannot find symbol llvm::ModulePass
> while loading mypass.so
>

I suspect you can't do that with gold, you need to instead link your pass
into the LLVM LTO plugin for Gold.

LLD may be able to load your pass though, I don't know it there is a
command line flag for this though.

-- 
Mehdi



> "/usr/bin/ld.gold" -pie --eh-frame-hdr -m elf_x86_64 -dynamic-linker
> /lib64/ld-linux-x86-64.so.2 -o test-instr
> /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/9.1.0/../../../../lib64/Scrt1.o
> /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/9.1.0/../../../../lib64/crti.o
> /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/9.1.0/crtbeginS.o
> -L/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/9.1.0
> -L/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/9.1.0/../../../../lib64
> -L/usr/bin/../lib64 -L/lib/../lib64 -L/usr/lib/../lib64
> -L/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/9.1.0/../../..
> -L/usr/bin/../lib -L/lib -L/usr/lib -plugin /usr/bin/../lib/LLVMgold.so
> -plugin-opt=mcpu=x86-64 -plugin-opt=O3 -plugin ../mypass.so
> /tmp/test-instr-4f23de.o -lgcc --as-needed -lgcc_s --no-as-needed -lc
> -lgcc --as-needed -lgcc_s --no-as-needed
> /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/9.1.0/crtendS.o
> /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/9.1.0/../../../../lib64/crtn.o
> /usr/bin/ld.gold: error: ../mypass.so: could not load plugin library:
> ../mypass.so: undefined symbol: _ZN4llvm10ModulePassD2Ev
> clang-8: error: linker command failed with exit code 1 (use -v to see
> invocation)
>
> The goal is using ḿodule, function, basic block iterators
> like before but just at a global scope.
>
> Any help/hint is appreciated.
> Many thanks and best regards, Heiko
>
>
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