Building llvm libc for an arbitrary RISC-V architecture string

[christian-herber-nxp]

RISC-V has many optional extensions, and I currently have no clue how to specify these in a libc build.
I have an easy to reproduce setup. After cloning the llvm project, I run this script from the root directory:

#!/usr/bin/env bash

rm -rf build-standalone
mkdir build-standalone
pushd build-standalone

C_COMPILER=clang-19
C_FLAGS="--target=riscv32 -march=rv32ic -mabi=ilp32"
CXX_COMPILER=clang++-19
TARGET_TRIPLE=riscv32-unknown-elf
cmake ../runtimes \
   -G Ninja \
   -DCMAKE_INSTALL_PREFIX=$(pwd)/install \
   -DCMAKE_C_COMPILER=$C_COMPILER \
   "-DCMAKE_C_FLAGS=${C_FLAGS}" \
   "-DCMAKE_CXX_FLAGS=${C_FLAGS}" \
   -DCMAKE_CXX_COMPILER=$CXX_COMPILER \
   -DLLVM_ENABLE_RUNTIMES=libc \
   -DLLVM_LIBC_FULL_BUILD=ON \
   -DLIBC_TARGET_TRIPLE=$TARGET_TRIPLE \
   -DCMAKE_BUILD_TYPE=Release
ninja libc libm
popd

Clang was installed with the following command:

wget -qO- | bash -s -- 19https://apt.llvm.org/llvm.sh
It fails with the following message:

    Run Build Command(s):/usr/bin/ninja cmTC_95cfc && [1/2] Building C object CMakeFiles/cmTC_95cfc.dir/testCCompiler.c.o
    [2/2] Linking C executable cmTC_95cfc
    FAILED: cmTC_95cfc 
    : && /usr/bin/clang-19 --target=riscv32 -march=rv32ic -mabi=ilp32  CMakeFiles/cmTC_95cfc.dir/testCCompiler.c.o -o cmTC_95cfc   && :
    ld.lld: error: unable to find library -lc
    ld.lld: error: unable to find library -lm
    ld.lld: error: unable to find library -lgcc

If you omit the CLAGS, it successfully builds for seemingly rv32ic.
Somebody knows how to do build for a user specified architecture string?

[llvmbot]

@llvm/issue-subscribers-libc

Author: Christian Herber (christian-herber-nxp)

RISC-V has many optional extensions, and I currently have no clue how to specify these in a libc build. I have an easy to reproduce setup. After cloning the llvm project, I run this script from the root directory:

#!/usr/bin/env bash

rm -rf build-standalone
mkdir build-standalone
pushd build-standalone

C_COMPILER=clang-19
C_FLAGS="--target=riscv32 -march=rv32ic -mabi=ilp32"
CXX_COMPILER=clang++-19
TARGET_TRIPLE=riscv32-unknown-elf
cmake ../runtimes \
   -G Ninja \
   -DCMAKE_INSTALL_PREFIX=$(pwd)/install \
   -DCMAKE_C_COMPILER=$C_COMPILER \
   "-DCMAKE_C_FLAGS=${C_FLAGS}" \
   "-DCMAKE_CXX_FLAGS=${C_FLAGS}" \
   -DCMAKE_CXX_COMPILER=$CXX_COMPILER \
   -DLLVM_ENABLE_RUNTIMES=libc \
   -DLLVM_LIBC_FULL_BUILD=ON \
   -DLIBC_TARGET_TRIPLE=$TARGET_TRIPLE \
   -DCMAKE_BUILD_TYPE=Release
ninja libc libm
popd

Clang was installed with the following command:

wget -qO- | bash -s -- 19https://apt.llvm.org/llvm.sh
It fails with the following message:

    Run Build Command(s):/usr/bin/ninja cmTC_95cfc && [1/2] Building C object CMakeFiles/cmTC_95cfc.dir/testCCompiler.c.o
    [2/2] Linking C executable cmTC_95cfc
    FAILED: cmTC_95cfc 
    : && /usr/bin/clang-19 --target=riscv32 -march=rv32ic -mabi=ilp32  CMakeFiles/cmTC_95cfc.dir/testCCompiler.c.o -o cmTC_95cfc   && :
    ld.lld: error: unable to find library -lc
    ld.lld: error: unable to find library -lm
    ld.lld: error: unable to find library -lgcc

If you omit the CLAGS, it successfully builds for seemingly rv32ic.
Somebody knows how to do build for a user specified architecture string?

[nickdesaulniers]

cc @mikhailramalho

[lntue]

instead of C_FLAGS or CMAKE_C_FLAGS/CMAKE_CXX_FLAGS, can you try -DLIBC_COMPILE_OPTIONS_DEFAULT="..."?

[christian-herber-nxp]

instead of C_FLAGS or CMAKE_C_FLAGS/CMAKE_CXX_FLAGS, can you try -DLIBC_COMPILE_OPTIONS_DEFAULT="..."?

this compiles, but changing the architecture flags does not seem to have an effect.

[nickdesaulniers]

What was your cmake invocation?

[christian-herber-nxp]

What was your cmake invocation?

i tried this:

C_FLAGS="--target=riscv32 -march=rv32imb_zce_zbkb -mabi=ilp32"
TARGET_TRIPLE=riscv32-unknown-elf
cmake ../runtimes \
   -G Ninja \
   -DCMAKE_INSTALL_PREFIX=$(pwd)/install \
   -DCMAKE_C_COMPILER=$C_COMPILER \
   "-DLIBC_COMPILE_OPTIONS_DEFAULT=${C_FLAGS}" \
   -DCMAKE_CXX_COMPILER=$CXX_COMPILER \
   -DLLVM_ENABLE_RUNTIMES=libc \
   -DLLVM_LIBC_FULL_BUILD=ON \
   -DLIBC_TARGET_TRIPLE=$TARGET_TRIPLE \
   -DCMAKE_BUILD_TYPE=Release
ninja libc libm

which compiles, but seems to not use the march flags.

this is the full output

-- Performing standalone runtimes build.
-- The C compiler identification is Clang 19.1.7
-- The CXX compiler identification is Clang 19.1.7
-- The ASM compiler identification is Clang with GNU-like command-line
-- Found assembler: /usr/bin/clang-19
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/clang-19 - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/clang++-19 - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Could NOT find LLVM (missing: LLVM_DIR)
-- Could NOT find Clang (missing: Clang_DIR)
-- Performing Test CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG
-- Performing Test CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG - Success
-- Performing Test C_SUPPORTS_START_NO_UNUSED_ARGUMENTS
-- Performing Test C_SUPPORTS_START_NO_UNUSED_ARGUMENTS - Success
-- Performing Test CXX_SUPPORTS_NOSTDLIBXX_FLAG
-- Performing Test CXX_SUPPORTS_NOSTDLIBXX_FLAG - Success
-- Performing Test CXX_SUPPORTS_NOSTDINCXX_FLAG
-- Performing Test CXX_SUPPORTS_NOSTDINCXX_FLAG - Success
-- Linker detection: GNU ld
-- Performing Test SUPPORTS_FVISIBILITY_INLINES_HIDDEN_FLAG
-- Performing Test SUPPORTS_FVISIBILITY_INLINES_HIDDEN_FLAG - Success
-- Performing Test CXX_SUPPORTS_SUGGEST_OVERRIDE_FLAG
-- Performing Test CXX_SUPPORTS_SUGGEST_OVERRIDE_FLAG - Success
-- Performing Test CXX_WSUGGEST_OVERRIDE_ALLOWS_ONLY_FINAL
-- Performing Test CXX_WSUGGEST_OVERRIDE_ALLOWS_ONLY_FINAL - Success
-- Performing Test C_WCOMMENT_ALLOWS_LINE_WRAP
-- Performing Test C_WCOMMENT_ALLOWS_LINE_WRAP - Success
-- Performing Test C_SUPPORTS_MISLEADING_INDENTATION_FLAG
-- Performing Test C_SUPPORTS_MISLEADING_INDENTATION_FLAG - Success
-- Performing Test CXX_SUPPORTS_MISLEADING_INDENTATION_FLAG
-- Performing Test CXX_SUPPORTS_MISLEADING_INDENTATION_FLAG - Success
-- Performing Test C_SUPPORTS_CTAD_MAYBE_UNSPPORTED_FLAG
-- Performing Test C_SUPPORTS_CTAD_MAYBE_UNSPPORTED_FLAG - Success
-- Performing Test CXX_SUPPORTS_CTAD_MAYBE_UNSPPORTED_FLAG
-- Performing Test CXX_SUPPORTS_CTAD_MAYBE_UNSPPORTED_FLAG - Success
-- Performing Test LINKER_SUPPORTS_COLOR_DIAGNOSTICS
-- Performing Test LINKER_SUPPORTS_COLOR_DIAGNOSTICS - Failed
-- Looking for os_signpost_interval_begin
-- Looking for os_signpost_interval_begin - not found
-- Found Python3: /usr/bin/python3.10 (found version "3.10.12") found components: Interpreter 
-- LLVM host triple: x86_64-unknown-linux-gnu
-- LLVM default target triple: x86_64-unknown-linux-gnu
-- Setting LIBC_NAMESPACE namespace to '__llvm_libc_21_0_0_git'
-- Set COMPILER_RESOURCE_DIR to /usr/lib/llvm-19/lib/clang/19 using --print-resource-dir
-- Building libc for riscv on baremetal with
        LIBC_COMPILE_OPTIONS_DEFAULT: --target=riscv32 -march=rv32imb_zce_zbkb -mabi=ilp32;--target=riscv32-unknown-elf
-- LIBC_CONF_ENABLE_STRONG_STACK_PROTECTOR: ON
-- LIBC_CONF_KEEP_FRAME_POINTER: ON
-- LIBC_CONF_ERRNO_MODE: LIBC_ERRNO_MODE_DEFAULT
-- LIBC_ADD_NULL_CHECKS: ON
-- LIBC_CONF_FREXP_INF_NAN_EXPONENT: 
-- LIBC_CONF_MATH_OPTIMIZATIONS: 0
-- LIBC_CONF_PRINTF_DISABLE_FIXED_POINT: OFF
-- LIBC_CONF_PRINTF_DISABLE_FLOAT: OFF
-- LIBC_CONF_PRINTF_DISABLE_INDEX_MODE: OFF
-- LIBC_CONF_PRINTF_DISABLE_STRERROR: OFF
-- LIBC_CONF_PRINTF_DISABLE_WRITE_INT: OFF
-- LIBC_CONF_PRINTF_FLOAT_TO_STR_NO_SPECIALIZE_LD: OFF
-- LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_DYADIC_FLOAT: OFF
-- LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_MEGA_LONG_DOUBLE_TABLE: OFF
-- LIBC_CONF_RAW_MUTEX_DEFAULT_SPIN_COUNT: 100
-- LIBC_CONF_RWLOCK_DEFAULT_SPIN_COUNT: 100
-- LIBC_CONF_TIMEOUT_ENSURE_MONOTONICITY: ON
-- LIBC_CONF_QSORT_IMPL: LIBC_QSORT_QUICK_SORT
-- LIBC_CONF_SCANF_DISABLE_FLOAT: OFF
-- LIBC_CONF_SCANF_DISABLE_INDEX_MODE: OFF
-- LIBC_CONF_SETJMP_AARCH64_RESTORE_PLATFORM_REGISTER: ON
-- LIBC_CONF_MEMSET_X86_USE_SOFTWARE_PREFETCHING: OFF
-- LIBC_CONF_STRING_UNSAFE_WIDE_READ: OFF
-- LIBC_CONF_TIME_64BIT: OFF
-- Writing config doc to /home/nxa23603/llvm-project/libc/docs/configure.rst
-- Overriding - LIBC_CONF_ERRNO_MODE: LIBC_ERRNO_MODE_EXTERNAL (Previous value: LIBC_ERRNO_MODE_DEFAULT)
-- Overriding - LIBC_CONF_MATH_OPTIMIZATIONS: (LIBC_MATH_SKIP_ACCURATE_PASS | LIBC_MATH_SMALL_TABLES) (Previous value: 0)
-- Overriding - LIBC_CONF_PRINTF_DISABLE_FLOAT: ON (Previous value: OFF)
-- Overriding - LIBC_CONF_PRINTF_DISABLE_INDEX_MODE: ON (Previous value: OFF)
-- Overriding - LIBC_CONF_PRINTF_DISABLE_STRERROR: ON (Previous value: OFF)
-- Overriding - LIBC_CONF_PRINTF_DISABLE_WRITE_INT: ON (Previous value: OFF)
-- Overriding - LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_MEGA_LONG_DOUBLE_TABLE: OFF (Previous value: OFF)
-- Overriding - LIBC_CONF_QSORT_IMPL: LIBC_QSORT_HEAP_SORT (Previous value: LIBC_QSORT_QUICK_SORT)
-- Set CPU features: 
-- Compiler features available: cfloat128;fixed_point;float128
-- Performing Test LIBC_CC_SUPPORTS_PATTERN_INIT
-- Performing Test LIBC_CC_SUPPORTS_PATTERN_INIT - Success
-- Performing Test LIBC_CC_SUPPORTS_NOSTDLIBPP
-- Performing Test LIBC_CC_SUPPORTS_NOSTDLIBPP - Success
-- Performing Test LIBC_CC_SUPPORTS_NOSTDLIBINC
-- Performing Test LIBC_CC_SUPPORTS_NOSTDLIBINC - Success
-- Integration test for hdrgen added.
-- check-runtimes does nothing.
-- Configuring done
-- Generating done

[mikhailramalho]

Hey @christian-herber-nxp, maybe you're missing the rv toolchain?

I use the one from here: https://github.com/riscv-collab/riscv-gnu-toolchain, and then I add --sysroot=/opt/riscv/sysroot --gcc-toolchain=/opt/riscv to DCMAKE_C_FLAGS/DCMAKE_CXX_FLAGS.

You can also check the full cmdline we use on the rv32 buildbot here: https://github.com/llvm/llvm-zorg/blob/16fd2d27482e8a8f240aad3396726a2776fdc324/zorg/buildbot/builders/annotated/libc-linux.py#L103

Let me know it works for you

[christian-herber-nxp]

Hey @christian-herber-nxp, maybe you're missing the rv toolchain?

I use the one from here: https://github.com/riscv-collab/riscv-gnu-toolchain, and then I add --sysroot=/opt/riscv/sysroot --gcc-toolchain=/opt/riscv to DCMAKE_C_FLAGS/DCMAKE_CXX_FLAGS.

You can also check the full cmdline we use on the rv32 buildbot here: https://github.com/llvm/llvm-zorg/blob/16fd2d27482e8a8f240aad3396726a2776fdc324/zorg/buildbot/builders/annotated/libc-linux.py#L103

Let me know it works for you

thanks, that was useful. I managed to compile. This is my final working solution for anybody else struggling:

TARGET_TRIPLE=riscv32-unknown-elf
C_FLAGS="--target=${TARGET_TRIPLE} -march=rv32imab_zce_zbkb --sysroot=${RISCV_GNU_TOOLCHAIN}/${TARGET_TRIPLE} --gcc-toolchain=${RISCV_GNU_TOOLCHAIN}"
cmake ../runtimes \
   -G Ninja \
   -DCMAKE_INSTALL_PREFIX=$(pwd)/install \
   -DCMAKE_C_COMPILER=$C_COMPILER \
   "-DCMAKE_C_FLAGS=${C_FLAGS}" \
   "-DCMAKE_CXX_FLAGS=${C_FLAGS}" \
   -DCMAKE_CXX_COMPILER=$CXX_COMPILER \
   -DLLVM_ENABLE_RUNTIMES=libc \
   -DLLVM_LIBC_FULL_BUILD=ON \
   -DLIBC_TARGET_TRIPLE=$TARGET_TRIPLE \
   -DCMAKE_BUILD_TYPE=Release
ninja libc libm

the fact that i have to have a gnu toolchain was a bit surprising to me. what is it used for? E.g. without the sysroot, it complains about missing crt0.s. Why would you need that to build libc?

Also, the A extension seems to be mandatory. Not having it generates this error:

/home/nxa23603/llvm-project/libc/src/__support/CPP/atomic.h:119:5: error: large atomic operation may incur significant performance penalty; the access size (4 bytes) exceeds the max lock-free size (0  bytes) [-Werror,-Watomic-alignment]
  119 |     __scoped_atomic_store(addressof(val), addressof(rhs), order(mem_ord),
      |     ^

Is the A extension really needed for building libc?

Aside from this, I noticed that the build will also fail if e.g. compiling for rv32imc_zaamo_zalrsc, which is basically equivalent to rv32imac, but this also fails. I will file a separate issue on that.

[christian-herber-nxp]

created #126706

[mikhailramalho]

the fact that i have to have a gnu toolchain was a bit surprising to me. what is it used for? E.g. without the sysroot, it complains about missing crt0.s. Why would you need that to build libc?

IIRC you only need crt if you are doing a full build, but I let the others answer that as I don't remember the details exactly.

Also, the A extension seems to be mandatory. Not having it generates this error:

/home/nxa23603/llvm-project/libc/src/__support/CPP/atomic.h:119:5: error: large atomic operation may incur significant performance penalty; the access size (4 bytes) exceeds the max lock-free size (0  bytes) [-Werror,-Watomic-alignment]
  119 |     __scoped_atomic_store(addressof(val), addressof(rhs), order(mem_ord),
      |     ^

Is the A extension really needed for building libc?

I think rand uses atomics

Aside from this, I noticed that the build will also fail if e.g. compiling for rv32imc_zaamo_zalrsc, which is basically equivalent to rv32imac, but this also fails. I will file a separate issue on that.

I commented there, it seems to be a warning changed into an error by using werror