On 5/9/22 15:18, Alexey Kardashevskiy wrote:

On 5/4/22 07:21, Nick Desaulniers wrote:

quoted

On Thu, Apr 28, 2022 at 11:46 PM Alexey Kardashevskiy [off-list ref] 
wrote:

quoted

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/ (local)

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,