llvm dev - Jul 2021 - Building compiler-rt for RISC-V?

My overall goal is locally building [1] an LLVM toolchain which compiles a
mixture of C and supporting assembler for bare RISC-V development
environments (no OS). Library support is required for some language
features on some RISC-V targets, e.g. integer division when the "M"
extension isn't present.
>From the details on https://compiler-rt.llvm.org/ I expected that something
like this on release/12.x would build the support library for RISC-V:

cmake -G Ninja ../llvm -DCMAKE_BUILD_TYPE=Release
-DLLVM_ENABLE_PROJECTS='clang;lld;compiler-rt'
-DCMAKE_INSTALL_PREFIX=~/bin/s4e -DLLVM_TARGETS_TO_BUILD='RISCV'
ninja
ninja install

however although that yields the RISCV compiler target libraries
(libLLVMRISCV*.a) it doesn't install any obvious RISCV runtime libraries
(only x86 ones) and linking a test program with integer division fails:

clang  testdiv.c --target=riscv32 -march=rv32i
ld.lld: error: unable to find library -lc
ld.lld: error: unable to find library -lm
ld.lld: error: unable to find library -lclang_rt.builtins-riscv32

(using LLVM release/12.x on x86_64 release/12.x; also tried building all
targets and also copying the cmake line from a build triggered by
Phabricator for a RISC-V-related change to compiler-rt with the hope that
some of the additional defines would prove the solution)

Thanks in advance for whatever advice you can provide for building the
support libraries for RISC-V.

Tom Goodfellow


[1] in the very short-term obtaining working ones from another build would
suffice, however I'll be needing to modify them soon anyway.
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On Tue, Jul 27, 2021 at 3:51 AM Thomas Goodfellow via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> My overall goal is locally building [1] an LLVM toolchain which compiles a
> mixture of C and supporting assembler for bare RISC-V development
> environments (no OS). Library support is required for some language
> features on some RISC-V targets, e.g. integer division when the
"M"
> extension isn't present.
>
> From the details on https://compiler-rt.llvm.org/ I expected that
> something like this on release/12.x would build the support library for
> RISC-V:
>
> cmake -G Ninja ../llvm -DCMAKE_BUILD_TYPE=Release
> -DLLVM_ENABLE_PROJECTS='clang;lld;compiler-rt'
> -DCMAKE_INSTALL_PREFIX=~/bin/s4e -DLLVM_TARGETS_TO_BUILD='RISCV'
> ninja
> ninja install
>
> however although that yields the RISCV compiler target libraries
> (libLLVMRISCV*.a) it doesn't install any obvious RISCV runtime
libraries
> (only x86 ones) and linking a test program with integer division fails:
>
> clang  testdiv.c --target=riscv32 -march=rv32i
> ld.lld: error: unable to find library -lc
> ld.lld: error: unable to find library -lm
> ld.lld: error: unable to find library -lclang_rt.builtins-riscv32
>
> (using LLVM release/12.x on x86_64 release/12.x; also tried building all
> targets and also copying the cmake line from a build triggered by
> Phabricator for a RISC-V-related change to compiler-rt with the hope that
> some of the additional defines would prove the solution)
>
> Thanks in advance for whatever advice you can provide for building the
> support libraries for RISC-V.
>
> Tom Goodfellow
>
Your --target= is not correct.

Normally, if you use --target=aarch64-linux-gnu, the clang driver will pick
up crt1.o crti.o crtn.o libc libm from libc, crtbegin.o/crtend.o from
aarch64-linux-gnu-gcc.
The libc and gcc paths are detected by locating aarch64-linux-gnu-gcc.
riscv may be more strange on some systems due to the complex
multiarch/multilib scheme.
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On Tue, Jul 27, 2021 at 11:51 AM Thomas Goodfellow via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> however although that yields the RISCV compiler target libraries
(libLLVMRISCV*.a) it doesn't install any obvious RISCV runtime libraries
(only x86 ones) and linking a test program with integer division fails
When compiling compiler-rt standalone (instead of as part of the
overall LLVM build) I have in the past used the following for the
rv32i build:

FLAGS="--target=riscv32-unknown-elf -march=rv32i -mabi=ilp32
--gcc-toolchain=$GCC_TOOLCHAIN --sysroot=$SYSROOT"
TEST_FLAGS="$FLAGS"

cmake $LLVM/compiler-rt \
  -G "Ninja" \
  -DCOMPILER_RT_BUILD_BUILTINS=ON \
  -DCOMPILER_RT_BUILD_SANITIZERS=OFF \
  -DCOMPILER_RT_BUILD_XRAY=OFF \
  -DCOMPILER_RT_BUILD_LIBFUZZER=OFF \
  -DCOMPILER_RT_BUILD_PROFILE=OFF \
  -DCMAKE_C_COMPILER=$LLVM_BUILD/bin/clang \
  -DCMAKE_AR=$LLVM_BUILD/bin/llvm-ar \
  -DCMAKE_NM=$LLVM_BUILD/bin/llvm-nm \
  -DCMAKE_RANLIB=$LLVM_BUILD/bin/llvm-ranlib \
  -DCMAKE_C_COMPILER_TARGET="riscv32-unknown-elf" \
  -DCMAKE_ASM_COMPILER_TARGET="riscv32-unknown-elf" \
  -DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON \
  -DCMAKE_C_FLAGS="$FLAGS" \
  -DCMAKE_ASM_FLAGS="$FLAGS" \
  -DCOMPILER_RT_OS_DIR="baremetal" \
  -DCOMPILER_RT_BAREMETAL_BUILD=ON \
  -DCOMPILER_RT_INCLUDE_TESTS=ON \
  -DCOMPILER_RT_EMULATOR="qemu-riscv32 -L $SYSROOT" \
  -DCOMPILER_RT_TEST_COMPILER="$LLVM_BUILD/bin/clang" \
  -DCOMPILER_RT_TEST_COMPILER_CFLAGS="$TEST_FLAGS" \
  -DLLVM_CONFIG_PATH=$LLVM_CONFIG

For compiling Compiler-RT as part of an integrated LLVM build, IIRC
you want to add it to the runtimes list (LLVM_ENABLE_RUNTIMES) and not
as a subproject. You can use cmake cache files to configure the
runtimes, and LLVM/compiler-rt has a weird setup for parsing cmake
settings of the form RUNTIMES_${target}_*. Check the existing cmake
cache files.

I hope that helps.

Best,
Luís