[PATCH kernel] powerpc/llvm/lto: Allow LLVM LTO builds

[Alexey Kardashevskiy]

On 5/9/22 15:18, Alexey Kardashevskiy wrote:
> 
> 
> On 5/4/22 07:21, Nick Desaulniers wrote:
>> On Thu, Apr 28, 2022 at 11:46 PM Alexey Kardashevskiy <aik at ozlabs.ru> 
>> wrote:
>>>
>>> This enables LTO_CLANG builds on POWER with the upstream version of
>>> LLVM.
>>>
>>> LTO optimizes the output vmlinux binary and this may affect the FTP
>>> alternative section if alt branches use "bc" (Branch Conditional) which
>>> is limited by 16 bit offsets. This shows up in errors like:
>>>
>>> ld.lld: error: InputSection too large for range extension thunk 
>>> vmlinux.o:(__ftr_alt_97+0xF0)
>>>
>>> This works around the issue by replacing "bc" in FTR_SECTION_ELSE with
>>> "b" which allows 26 bit offsets.
>>>
>>> This catches the problem instructions in vmlinux.o before it LTO'ed:
>>>
>>> $ objdump -d -M raw -j __ftr_alt_97 vmlinux.o | egrep '\S+\s*\<bc\>'
>>>    30:   00 00 82 40     bc      4,eq,30 <__ftr_alt_97+0x30>
>>>    f0:   00 00 82 40     bc      4,eq,f0 <__ftr_alt_97+0xf0>
>>>
>>> This allows LTO builds for ppc64le_defconfig plus LTO options.
>>> Note that DYNAMIC_FTRACE/FUNCTION_TRACER is not supported by LTO builds
>>> but this is not POWERPC-specific.
>>
>> $ ARCH=powerpc make LLVM=1 -j72 ppc64le_defconfig
>> $ ARCH=powerpc make LLVM=1 -j72 menuconfig
>> <disable FTRACE, enable LTO_CLANG_THIN>
>> $ ARCH=powerpc make LLVM=1 -j72
>> ...
>>    VDSO64L arch/powerpc/kernel/vdso/vdso64.so.dbg
>> /usr/bin/powerpc64le-linux-gnu-ld:
>> /android0/llvm-project/llvm/build/bin/../lib/LLVMgold.so: error
>> loading plugin:
>> /android0/llvm-project/llvm/build/bin/../lib/LLVMgold.so: cannot open
>> shared object file: No such file or directory
>> clang-15: error: linker command failed with exit code 1 (use -v to see
>> invocation)
>> make[1]: *** [arch/powerpc/kernel/vdso/Makefile:67:
>> arch/powerpc/kernel/vdso/vdso64.so.dbg] Error 1
>>
>> Looks like LLD isn't being invoked correctly to link the vdso.
>> Probably need to revisit
>> https://lore.kernel.org/lkml/20200901222523.1941988-1-ndesaulniers@google.com/
>>
>> How were you working around this issue? Perhaps you built clang to
>> default to LLD? (there's a cmake option for that)
> 
> 
> What option is that? I only add  -DLLVM_ENABLE_LLD=ON  which (I think) 
> tells cmake to use lld to link the LLVM being built but does not seem to 
> tell what the built clang should do.
> 
> Without -DLLVM_ENABLE_LLD=ON, building just fails:
> 
> [fstn1-p1 ~/pbuild/llvm/llvm-lto-latest-cleanbuild]$ ninja -j 100
> [619/3501] Linking CXX executable bin/not
> FAILED: bin/not
> : && /usr/bin/clang++ -fPIC -fvisibility-inlines-hidden 
> -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra 
> -Wno-unused-parameter -Wwrite-strings -Wcast-qual 
> -Wmissing-field-initializers -pedantic -Wno-long-long 
> -Wc++98-compat-extra-semi -Wimplicit-fallthrough 
> -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor 
> -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion 
> -Wmisleading-indentation -fdiagnostics-color -ffunction-sections 
> -fdata-sections -flto -O3 -DNDEBUG -flto 
> -Wl,-rpath-link,/home/aik/pbuild/llvm/llvm-lto-latest-cleanbuild/./lib 
> -Wl,--gc-sections utils/not/CMakeFiles/not.dir/not.cpp.o -o bin/not 
> -Wl,-rpath,"\$ORIGIN/../lib"  -lpthread  lib/libLLVMSupport.a  -lrt 
> -ldl  -lpthread  -lm  /usr/lib/powerpc64le-linux-gnu/libz.so 
> /usr/lib/powerpc64le-linux-gnu/libtinfo.so  lib/libLLVMDemangle.a && :
> /usr/bin/ld: lib/libLLVMSupport.a: error adding symbols: archive has no 
> index; run ranlib to add one
> clang: error: linker command failed with exit code 1 (use -v to see 
> invocation)
> [701/3501] Building CXX object 
> utils/TableGen/CMakeFiles/llvm-tblgen.dir/GlobalISelEmitter.cpp.o
> ninja: build stopped: subcommand failed.
> 
> 
> 
> My head hurts :(
> The above example is running on PPC. Now I am trying x86 box:
> 

A bit of progress.

cmake -G Ninja -DLLVM_ENABLE_PROJECTS="clang;lld" 
-DLLVM_TARGET_ARCH=PowerPC -DLLVM_TARGETS_TO_BUILD=PowerPC 
~/llvm-project//llvm -DLLVM_ENABLE_LTO=ON 
-DLLVM_BINUTILS_INCDIR=/usr/lib/gcc/powerpc64le-linux-gnu/11/plugin/include/ 
-DCMAKE_BUILD_TYPE=Release

produces:

-- Native target architecture is PowerPC 

....
-- LLVM host triple: x86_64-unknown-linux-gnu
-- LLVM default target triple: x86_64-unknown-linux-gnu


and the resulting "clang" can only to "Target: 
x86_64-unknown-linux-gnu", how do you build LLVM exactly? Thanks,